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Title: Texas Gemstones
Author: King, Elbert A., Jr.
Language: English
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*** Start of this LibraryBlog Digital Book "Texas Gemstones" ***
BUREAU OF ECONOMIC GEOLOGY
The University of Texas at Austin
Austin, Texas 78712
JOHN T. LONSDALE, _Director_
Report of Investigations—No. 42
Texas Gemstones
By
Elbert A. King, Jr.
February 1961
_Second Printing—February 1963
Third Printing—September 1972
Fourth Printing—March 1983
Fifth Printing—August 1991_
Contents
Page
Introduction 5
Properties of gemstones 5
Crystals 7
Cutting and polishing of gemstones 10
Cabochon gems 10
Faceted gems 13
Tumbled gems 17
Texas gemstones 18
Amber 18
Augite 18
Beryl 18
Celestite 19
Diamond 19
Epidote 19
Fluorite 20
Fossil wood 20
Gadolinite 21
Garnet 22
Jet 22
Labradorite 23
Microcline 23
Obsidian 24
Opal 24
Pearl 24
Quartz 25
Crystalline varieties 25
Amethyst 25
Citrine 25
Rock crystal 26
Rose quartz 26
Smoky quartz 26
Cryptocrystalline varieties 27
Chalcedony 27
Agate 27
Agatized wood 27
Carnelian 27
Jasper 27
Sanidine 28
Spinel 28
Tektite (bediasite) 28
Topaz 29
Tourmaline 30
Turquoise 31
Glossary 32
Selected references 34
Index 41
Illustrations
Figures— Page
1. Typical crystal form of three common Texas gemstones 9
2. Variations of the cabochon cut 10
3. Diamond saw 11
4. Cabochon properly attached to dop-stick 12
5. Cabochons at various stages of cutting and polishing 12
6. Nomenclature of the standard American brilliant cut 13
7. Facet table 14
8. Grinding the table facet on a rough stone 15
9. Stone dopped to table facet 15
10. Preformed stone dopped to table facet 16
11. Proper sequence of cutting of the pavilion facets 16
12. Proper placing of pavilion girdle facets 17
13. Proper sequence of cutting of crown facets 17
14. Common crystal form of Travis County celestite 19
15. Common crystal form of fluorite 20
16. Crystal faces on microcline specimen shown in Plate III 23
17. Common crystal form of spinel 28
18. Crystal faces on topaz crystal shown in Plate V 29
19. Cross section through irregularly colored stone 30
20. Common crystal form of Llano County tourmaline 31
Plates— Page
I. A, Gem-quality celestite crystals from Travis County. B,
Opalized wood from the Texas Gulf Coastal Plain 35
II. A, Gem-quality garnet crystals and faceted gem from Gillespie
County. B, Labradorite from Brewster County 36
III. A, Pink microcline crystal. B, Smoky quartz. Both from Burnet
County 37
IV. Polished agate from gravels of the Rio Grande near Zapata,
Zapata County 38
V. A, Texas tektites (bediasites). B, Topaz crystal from a
pegmatite dike near Streeter, Mason County 39
VI. A, Topaz from stream gravels near Streeter, Mason County. B,
Tourmaline crystals in schist from Llano County 40
Table 1. Properties of some common Texas gem minerals 8
Texas Gemstones
ELBERT A. KING, JR.
INTRODUCTION
Throughout history man has sought stones and minerals for personal
adornment and ornamentation. Stones and minerals that are sufficiently
beautiful, durable, and rare are known as gemstones. A gemstone with
only one of these qualities is less desirable than one with all three.
For example, a stone with rich color but not sufficiently durable to
withstand daily wear in rings finds little favor as a gemstone except in
brooches or pins where the stone is relatively safe from abrasion.
Likewise, a stone that is beautiful and durable may be of little
interest as a gemstone because it is commonly found in great quantities.
To be valued highly, gemstones must be beautiful to the eye, durable
enough to withstand wear, and rare enough so that they are not easily
obtained.
Properties of Gemstones
The beauty of gemstones is mostly dependent on their color, diaphaneity,
brilliancy, luster, and fire. Any one or a combination of these
properties render stones desirable as gems.
Color is very important in many gemstones. The color of transparent
varieties should be distinct enough to be pleasing to the eye, yet not
so dark as to appear black or opaque. It is generally more desirable
that the gemstone be of even color and not appear “patchy” or
“streaked.” However, some opaque or translucent stones such as agate owe
their popularity chiefly to the variety of colors and designs within a
single piece. Some transparent gemstones exhibit different colors when
viewed in different directions. For example, some fine blood-red rubies
appear brownish when viewed in a particular direction. The gemstone
should be cut so that its finest color is most prominently displayed.
This ability of some gemstones to exhibit different colors when viewed
in different directions is called pleochroism.
Diaphaneity is the relative ability of stones to transmit light.
Diaphaneity is described by terms such as transparent, translucent, and
opaque. Transparency is highly desirable in stones such as diamond that
are commonly facet-cut to reflect light. The gemstone should be water
clear and free from inclusions and cracks so that it transmits light
freely, but there are stones that do not exhibit this property that are
prized as gemstones. For example, turquoise may appear to be completely
opaque and not transmit any light, but it is sought for its fine blue
color.
The brilliancy of gemstones is largely dependent on their index of
refraction. The index of refraction is a measure of the ability of a cut
gemstone to “bend” light rays and reflect them from the bottom facets
back through the top of the stone. Of course, brilliancy is not noted in
opaque or faintly translucent stones. The index of refraction of
gemstones is expressed numerically. Air is the reference medium and is
assigned an index of refraction of 1.00. Other substances are assigned
values relative to that of air, for example, water, 1.33; topaz, 1.62;
diamond, 2.42. The higher the index of refraction, the more brilliant
will be the gemstone if it is properly cut and polished.
Luster is the appearance of the mineral on a fresh surface in reflected
light; it is divided into two major categories, metallic and
non-metallic. Most gemstones have non-metallic luster and are described
by terms such as vitreous or glassy, resinous, waxy, greasy, and pearly.
The fire, or ability of gemstones to show flashes of different colors of
light, is dependent upon a property called dispersion. The amount of
dispersion is the extent to which the gemstone is able to separate
ordinary white light into its component colors. The dispersion of
gemstones can also be expressed numerically but for purposes of this
publication will be referred to as low, moderate, or high. Diamond is a
common gemstone that has high dispersion.
A gemstone’s durability is primarily dependent upon its hardness. The
Mohs scale of hardness, given below, is most commonly used for gemstones
and other minerals.
_Mohs Scale of Hardness_
1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond
On this scale, the higher numbers are the harder minerals. Mohs is a
relative, not an absolute scale. Therefore, it should not be assumed
that diamond is ten times harder than talc because actually diamond is
very many tens of times harder than talc. However, a particular mineral
is harder than any other mineral with a lesser number, and the scale is
very convenient to use. Gemstones mounted in rings should have a
hardness of at least seven on the Mohs scale, or the stones may become
scuffed and scratched after a relatively short period of wear. Gemstones
mounted in pins and brooches can be of softer material as they are not
usually subjected to abrasion and rough treatment.
The tendency of some minerals to split with relative ease in particular
directions along planes is called cleavage. Cleavage is also a factor
determining the durability of gemstones. Some gemstones do not exhibit
this tendency at all, whereas others cleave in several directions. The
number of cleavages is always the same in any one mineral, and the
direction of cleavages is constant in relation to the crystal structure
of any one mineral or gemstone. It is apparent that of stones having the
same hardness, the ones lacking cleavage or having the lesser number of
good cleavage directions are the most durable.
Some stones, such as jade and agate, owe their durability to their
compact fibrous structure, which makes them very tough and durable even
though they are not especially hard.
Several other properties of gemstones, although not always contributing
to the beauty or desirability of gemstones, are useful in identifying
uncut specimens.
Streak is the color of the mineral when finely powdered or, for softer
minerals, the color obtained by rubbing the mineral against a piece of
unglazed porcelain or tile. The color of a mineral’s streak is commonly
different from the unpowdered specimen.
Fracture is the kind of surface obtained when the mineral is broken in a
direction that is not a cleavage direction. Fracture surfaces are
described by such terms as conchoidal (like the fracture of glass),
subconchoidal, splintery, even, and uneven.
Tenacity is the resistance of a mineral to breakage. Brittle minerals
break relatively easily on impact. Malleable minerals, such as gold, may
be flattened under a hammer into very thin sheets without breaking.
Sectile minerals may be cut with a knife without powdering. Most
gemstones, even diamond, are brittle.
It is only natural to value most those gemstones that are not common or
easy to obtain. Emerald owes its longstanding popularity to its fine
green color, but tourmaline is sometimes found in colors that very
closely approach that of emerald and yet sells for considerably less
because it is so much more common.
Rarity is not the only factor affecting the value of gemstones. Freedom
from internal imperfections, quality of cutting, color, and size must
also be considered in cut and polished gemstones. Internal
imperfections, such as inclusions and cracks, detract from the
appearance of gemstones and interfere with the passage of light between
the facets; consequently, gemstones containing these imperfections are
not valued as highly as those without them. Poor cutting or polishing
detract from the beauty and thus from the value of gemstones. Unpopular
or poor color commonly causes gemstones to be less valuable. Rich green
emeralds are exceedingly prized, whereas very pale green emeralds are
relatively inexpensive. Diamonds that have the least hint of yellow are
never valued as highly as pure colorless, pink, or blue stones. Few
persons find the yellowish color attractive, unless it is a vivid canary
yellow.
Size is important in determining the value of gemstones but not as
important as perfection. A badly flawed gemstone of large size may be
worth only a slight fraction of the value of a smaller perfect one.
Gemstone size is usually measured in carats, a unit of weight, although
millimeter size is sometimes used. Five carats is equal to 1 gram and
approximately 28⅓ grams is equal to 1 ounce avoirdupois. One
one-hundredth (0.01) of a carat is called a point, and this term is
often used, especially pertaining to very small gemstones.
The term used to compare the relative weights of minerals and gemstones
is specific gravity, which is expressed numerically in relation to
water. Water is assigned the value of 1.00. Therefore, at a given
temperature a gemstone having a specific gravity of 2.00 is twice as
heavy as an equal volume of water. A 1-carat sapphire (specific gravity
about 4.00) will be smaller than a 1-carat amethyst (specific gravity
about 2.65) because the heavier material will occupy less volume to have
the same weight.
A summary of properties helpful in identification of common Texas gem
minerals is given in Table 1.
Comparatively recently in the history of gemstones, man has succeeded in
the production of synthetic gems that have properties closely
approaching those of many natural gemstones. To the untrained eye some
synthetic gems may appear identical to natural stones, but synthetic
gems can be detected with little difficulty by a properly equipped
expert. Although most synthetic gems are inexpensive, their manufacture
has not adversely affected the value of natural gemstones but instead
has increased the demand for fine natural gems.
Crystals
Gemstones that have an orderly internal molecular arrangement are
referred to as crystalline. This internal order is commonly reflected in
the external shape of “rough” or uncut gemstones. The resultant shape is
a polyhedral solid bounded by planes and called a crystal. Well-formed
crystals are formed in nature only under relatively ideal conditions of
temperature, pressure, and space. The specific temperatures and
pressures involved vary with different minerals, but most crystals need
space in which to form so that their “growth” is not impaired by
surrounding rocks and minerals. However, some minerals, such as garnet
and tourmaline, can grow in metamorphic rocks by recrystallization of
minerals in the metamorphic rocks. The size of crystals varies from
microscopic to tens of feet. Any one mineral usually has one or two
typical crystal forms or arrangements of plane surfaces that aid greatly
in the identification of the mineral when it occurs in good crystals
(fig. 1). Frequently gemstones are found as abraded stream-rolled
pebbles, fragments, or masses that do not show crystal form. Crystals of
the same mineral from different locations commonly show somewhat
different crystal forms owing to slight differences in composition or
conditions of formation. Mineralogists and crystallographers classify
crystals by the symmetry that they exhibit. The crystal systems are (1)
isometric or cubic, (2) tetragonal, (3) hexagonal, (4) orthorhombic, (5)
monoclinic, and (6) triclinic. A complete description of the
classification of crystals can be found in almost any mineralogy text
(see Selected References, p. 34).
Table 1. Properties of some common Texas gem minerals.
MINERAL COMPOSITION HARDNESS SPECIFIC INDEX OF COMMON
GRAVITY REFRACTION COLORS IN
TEXAS
Amber fossil resin 2.0-2.5 1.05-1.10 about 1.54 brown, yellow
Augite CaMgSi₂O₆ 5.0-6.0 3.2-3.6 1.60-1.71 greenish
brown, black
Beryl Be₃Al₂(SiO)₆ 7.5-8.0 2.63-2.80 1.56-1.60 pale blue,
colorless,
greenish
Celestite SrSO₄ 3.0-3.5 3.95-3.98 1.62-1.63 colorless,
blue
Epidote HCa₂(Al, 6.0-7.0 3.25-3.50 1.72-1.77 yellowish
Fe)₃Si₃O₁₃ green,
brownish
green
Fluorite CaF₂ 4.0 3.0-3.25 1.434 colorless,
violet,
yellow, green
Garnet Fe₃Al₂(SiO₄)₃ about 7.5 4.25 about 1.83 red, deep
(Almandite) red,
brownish red
Labradorite NaAlSi₃O₈ 50% to 6.0-6.5 about 2.6 about 1.56 yellowish,
30% CaAlSi₃O₈ 50% grayish
to 70%
Microcline KAlSi₃O₈ 6.0-6.5 2.54-2.57 1.52-1.53 pink, red,
bluish,
greenish,
white
Obsidian volcanic glass 5.0-5.5 2.3-2.5 1.45-1.53 dark gray,
black,
brownish
Opal SiO₂·nH₂O 5.5-6.5 1.9-2.3 1.43 white, pink,
bluish,
brown, gray
Quartz SiO₂ 7.0 2.65-2.66 1.544-1.553 colorless,
(Crystalline) violet,
yellow, brown
Tektite natural glass 5-6 2.33-2.44 1.48-1.52 dark brown,
(Bediasite) greenish
brown
Topaz Al₂(F·OH)₂SiO₄ 8.0 3.4-3.6 1.60-1.63 colorless,
bluish, sky
blue
Tourmaline H₉Al₃(B·OH)₂Si₄O₁₉ 7.0-7.5 2.98-3.20 1.62-1.64 black, dark
brown
Some gemstones, such as opal and obsidian, never occur as crystals owing
to a lack of internal structural order. Such gemstones are termed
amorphous, or without form. Amorphous gemstones mostly occur in nature
as irregular lumps or masses, cavity fillings, or veins.
[Illustration: Fig. 1. Typical crystal form of three common Texas
gemstones.]
GARNET
TOURMALINE
QUARTZ
CUTTING AND POLISHING OF GEMSTONES
There are two types of widely used gemstone cuts. Opaque or figured
gemstones are usually cut with a rounded upper surface and a flat or
rounded back. A stone cut in this fashion is termed a cabochon or is
said to be cabochon cut. There are several variations of this mode of
cutting (fig. 2). Precious opal, agate, jade, star sapphire, and fossil
wood are some of the stones that are cut mostly as cabochons.
Transparent gemstones are usually cut with many plane polished surfaces.
Such stones are called faceted, and the process of cutting and polishing
these stones is called faceting. Emerald, diamond, topaz, and garnet are
examples of gemstones that are commonly seen as faceted stones.
[Illustration: Fig. 2. Variations of the cabochon cut. Left to right:
double cabochon; flat cabochon; simple cabochon; hollow cabochon.]
The cutting of gemstones, although sometimes tedious and time consuming,
is not especially difficult or complex. However, like most arts and
crafts, technique and ability should improve with practice and
experience. There are currently many amateur gem cutters in Texas. A
complete set of equipment necessary to cut cabochon stones may be
purchased for as little as $50.00 or $60.00. Most amateur cabochon
cutters have equipment that cost less than $100.00 which enables them to
do very fine work on many gem materials. Facet cutting requires more
precise equipment, and a complete array of such usually costs more than
$100.00, although less expensive equipment can be obtained. The
beginning gem cutter or lapidary who is willing to assemble and make
some of his own equipment can reduce his initial expenses considerably.
Cabochon Gems
The procedures listed herein for gem cutting do not apply to all
gemstones. Stones that are especially brittle, soft, or difficult to
polish require additional procedures or special techniques. Many
lapidaries may deviate from these procedures. Some of the steps of
cutting and polishing are merely matters of personal opinion and vary
somewhat from cutter to cutter. There are several detailed texts on the
art of gem cutting; the descriptions herein are designed to give the
reader only a general idea of the procedures and techniques involved.
The cutting and polishing of cabochons require several steps. The
initial step is sawing. Assuming that the rough gem material is large
enough to be sawed (larger than about half an inch in diameter), it is
clamped into the carriage of a diamond saw (fig. 3) and cut into slices
about ⅜-inch thick. The blade of the saw is mild steel that has been
impregnated with diamond dust around the edge, hence the name diamond
saw. The blade is rotated rapidly, and the material to be cut is “fed”
to the blade by a sliding carriage on which the gem material is clamped.
The extreme hardness of the diamond dust in the edge of the blade
enables the saw to cut through several inches of gem material in a few
minutes. The lower portion of the saw blade is immersed in a mixture of
kerosene and oil, and the rotating saw blade carries with it some of the
kerosene-oil mixture; this acts as a coolant and lubricant for both the
saw blade and the material being cut. Without this lubricant, the heat
generated by sawing would shatter most gem materials and also damage the
saw blade. As this “slicing” or sawing of the material usually takes
several minutes, a weight and pulley are generally used to give the gem
material the necessary pressure against the saw blade. When cut through,
the “slab” of gem material falls into the kerosene-oil mixture at the
bottom of the saw or onto a special platform that cushions its fall.
[Illustration: Fig. 3. Diamond saw.]
Motor
Clamp
Diamond-charged blade
Carriage
Stone
Weight
After being sawed, the slab of gem material is examined, and the
location and size of the stones to be cut from the slab are determined.
The desired outline of the shape of the gem to be cut is marked on the
slab with a pointed piece of aluminum rod; ordinary pencil marks are not
used because they wear away too quickly in the cutting process. Once the
area from which the gem is to be cut has been selected and the outline
of the gemstone has been marked on the slab, the excess material is
trimmed away by a smaller diamond saw known as a trim-saw. In some slabs
the excess material can be broken and “nibbled” away with a strong pair
of pliers.
The remaining portion of the stone is usually held by hand and ground to
the desired shape using the previously scribed mark as a guide. This is
done using a relatively coarse-grained (about 150 grit) specially made
carborundum grinding wheel.
Now that the desired outline has been obtained, the stone is firmly
affixed to a slender wooden or hollow aluminum dop-stick (fig. 4). The
process whereby the stone is attached to the dop-stick with a specially
compounded jeweler’s wax is called dopping. The dop-wax is heated over
an alcohol lamp or candle flame until it is soft and pliable and is then
spread around on the end of the dop-stick and formed into a mass about
the right size and shape to fit the back of the gemstone. The stone is
likewise heated, and the wax is applied to the back of the stone while
both wax and stone are hot. Upon cooling, the wax firmly fixes the stone
to the dop-stick. The dop-stick allows the lapidary to have firm control
of the stone during all later stages of cutting and polishing.
[Illustration: Fig. 4. Cabochon properly attached to dop-stick.]
CABOCHON
DOP-WAX
DOP-STICK
The top of the dopped gemstone is worked against the coarse carborundum
grinding wheel until it is a rough approximation of the desired shape.
The stone is then worked against a much finer-grained (about 220 grit)
grinding wheel to remove the irregularities left by the coarse grinding
and to further smooth and shape the surface of the gemstone. At all
times while grinding, a small flow of water should be directed on the
grinding wheel to keep the stone cool. Grinding on the stone for even a
few minutes without cooling may result in the shattering of the gemstone
because of heat created by friction of the stone against the grinding
wheel. If the lapidary keeps the surface of the grinding wheel wet,
there is little chance of damaging most gem materials.
The next phase of cabochon cutting and polishing is sanding. The
gemstone is worked against two sanding drums of different grit size.
This sanding can be done with the sandpaper surface either wet or dry,
as needed or as preferred by the lapidary. However, great care should be
exercised during sanding so that the stone is not overheated.
Overheating can easily occur whether the sandpaper is used wet or dry.
As in grinding, sanding is first done on coarser grit paper (about 300
grit) and last on finer paper (about 600 grit). It is in the sanding
process that the first hint of polish is noted on the surface of the
stone. After sanding, the gemstone should have perfect form with no
surface irregularities, a very finely textured surface, and only very
minor scratches left from sanding. The gemstone is now ready to be
polished.
[Illustration: Fig. 5. Cabochons at various stages of cutting and
polishing. Left to right: trimmed from slab: ground to outline; after
rough grinding; after sanding; polished.]
At this point the procedure depends on the nature of the gemstone being
polished. Most gem materials are worked against a buffing wheel that is
impregnated or saturated with a mixture of some polishing compound and
water. A soft felt buffing wheel with cerium oxide as the polishing
agent is used for many materials. The mixture of cerium oxide and water
is usually applied to the buffing wheel with a small brush. The lapidary
should once more be careful not to overheat the stone. If the stone
becomes too hot to hold to the underside of the cutter’s wrist, it
should be permitted to cool for a few seconds before continuing. After
polishing on the buffing wheel, the gemstone should have a fine, high
polish and be free of any scratches or surface irregularities. The
finished gemstone is removed from the dop-stick by heating the dop-wax
and pulling the stone loose. Any excess wax that hardens again before it
can be removed from the stone by hand can be dissolved away by rubbing
with an acetone-soaked cloth. Figure 5 illustrates the desired
appearance of the gemstone at the end of each of the steps of cutting
and polishing.
Faceted Gems
The principles involved in faceting are about the same as those in the
cutting of cabochons, but the equipment and technique are considerably
different. The equipment required for the facet cutting of gemstones is
built into or attached to a small specially constructed table (fig. 7),
and the unit is commonly called a facet table. Most faceted gemstones
are cut to obtain the largest flawless stone possible from the rough
material. Therefore, one of the first and most important steps for the
lapidary is to decide how the stone is to be cut from the rough crystal
or pebble. The colors that can be obtained from the gemstone must also
be considered, and the cutting of the stone oriented so that its best
color is displayed. The lapidary also selects the orientation of the
stone in relation to the cleavage or cleavages. It is difficult or
impossible to polish facets of gemstones that are cut parallel to a good
cleavage direction.
[Illustration: Fig. 6. Nomenclature of the standard American brilliant
cut.]
TOP VIEW
SIDE VIEW
Star facet
Crown main facet
Crown girdle facet
Pavilion girdle facet
Pavilion main facet
TABLE
CROWN GIRDLE
PAVILION
CULET
BOTTOM VIEW
Once the orientation of the gemstone to be cut from the rough material
has been determined, the stone is dopped onto a special metal dop-stick
that fits into the chuck of the facet head. The chuck is tightened so
that the position of the stone on the end of the arm of the facet head
is firmly fixed, and the facet head is adjusted so that the first facet
that is cut is the horizontal, top facet of the stone or table facet
(fig. 6). The table facet is cut by grinding the gemstone on a flat
cutting lap that is diamond impregnated (fig. 8). By minor adjustments
of the facet head, the lapidary can precisely control the location of
the table facet. As soon as the table facet has been ground to the
proper size, the cutting lap is removed from the lap plate, and the
polishing lap is secured in place. Many different kinds of polishing
laps and polishing compounds may be used depending on the properties of
the material being polished. However, one lap and one polishing compound
are usually sufficient for each gem variety. After the polishing lap is
secured to the lap plate, the lapidary adjusts the facet head so that
the stone is in exactly the same position relative to the lap that it
was during the cutting of the table facet. The polishing lap is run wet
or damp with water, as is the cutting lap, and small amounts of the
polishing compound are applied to the surface of the lap while the facet
is being polished. The minor scratches left by the cutting process are
gradually removed, and a fine lustrous polish develops on the facet. It
is especially important to take care in achieving a perfect polish on
the table facet, as this facet occupies a large area of the crown of the
gemstone. When the cutting and polishing of the table facet are
completed, the gemstone is still rough or uncut in all portions except
for this single, large, polished surface.
[Illustration: Fig. 7. Facet table.]
Water
Light
Adjusting ring
Post
Arm
Chuck
Stone
Abrasives
DIAMOND DUST
CALCIUM OXIDE
LANDE-A
[Illustration: Fig. 8. Grinding the table facet on a rough stone.]
CHUCK
DOP-STICK
DOP-WAX
STONE
LAP
The gemstone is then removed from the dop-stick by melting the dop-wax
and is dopped once more so that the plane of the polished table facet is
perpendicular to the axis of the chuck and arm of the facet head (fig.
9). Great care should be taken by the lapidary to insure that the table
of the stone is exactly perpendicular to this axis, or the proper
placing of the later facets on the stone may become very difficult.
[Illustration: Fig. 9. Stone dopped to table facet.]
TABLE FACET
DOP-WAX
STONE
DOP-STICK
Once the stone has been properly dopped to the table facet, the lapidary
is ready to proceed with the cutting of the outline of the stone. If it
is to be a brilliant cut, the stone is ground perfectly round in
outline; if it is to be an emerald or step cut, it is shaped so that it
is square or rectangular in outline. This process is called preforming.
The arm of the facet head is lowered on the post until it is horizontal,
and the stone is worked against the cutting lap until the desired shape
is obtained. When the preforming process is completed, the stone should
have the desired outline of the finished gem (fig. 10).
[Illustration: Fig. 10. Preformed stone dopped to table facet.]
DOP-WAX
STONE
DOP-STICK
The lapidary is now ready to proceed with the cutting of the pavilion of
the stone. The arm of the facet head is raised to the proper angle for
cutting the main pavilion facets. The angle at which the main facets are
cut is very critical in determining the beauty of the finished stone.
The required angle at which these facets must be cut varies with the
refractive indices of the different varieties of gem minerals. If the
facets are not cut at exactly the proper angle, light entering the top
or crown of the gemstone can pass completely through the stone, instead
of being reflected back out of the crown facets. The result is a dull,
lifeless stone that appears to have a “hole” or “fish-eye” in the
center. Stones that are cut in this manner are greatly reduced in value.
The angle at which the facets are cut is controlled by the adjustment of
the height of the arm of the facet head on the post. The lapidary will
continually adjust this height, because the angle between the arm and
the surface of the lap changes slightly as the facet is ground down to
its proper place and size.
[Illustration: Fig. 11. Proper sequence of cutting of the pavilion
facets. Left to right: four main facets; all eight main facets; half of
the pavilion girdle facets; completed pavilion.]
The standard American brilliant cut will be used as an example of facet
cutting. Procedure for all other cuts is essentially the same to this
point. After the eight main pavilion facets have been cut, the cutting
angle is changed a few degrees, the arm of the facet head rotated
slightly, and the sixteen pavilion girdle facets or “skill” facets, as
they are often called, are cut (fig. 11). The pavilion girdle facets
should meet exactly in the center of the main facets at the girdle of
the stone. The pavilion girdle facets should neither overlap, nor should
there be any space between them (fig. 12). After the pavilion girdle
facets are cut, the cutting of the pavilion of the gemstone is
completed. The facets are then polished on the polishing lap at the same
angles and in the same order as they were cut, and the pavilion of the
gem is completely finished.
The stone is then removed from the dop-stick by melting the dop-wax and
is re-dopped to the pavilion facets so that the crown of the stone is
now exposed for cutting. Before the lapidary proceeds with the cutting
of the crown, it is necessary that the stone be perfectly centered on
the dop-stick and that the plane of the table facet be perpendicular to
the dop-stick and to the axis of the arm of the facet head. The eight
main facets are cut first, with numerous adjustments being made by the
lapidary to insure that the proper angle is maintained (fig. 13). Then
the cutting angle is changed a few degrees, the arm of the facet head
rotated slightly, and the crown girdle facets are cut. The crown girdle
facets are placed very similarly to the pavilion girdle facets except
that they are shorter. The crown girdle facets should be joined in
exactly the same way as the pavilion girdle facets. When these facets
are properly cut, the cutting angle is again changed, the arm rotated,
and the eight star facets are cut. This completes the cutting of the
crown of the stone. The cutting lap is removed from the lap plate, and
the polishing lap is secured into place. The facets are carefully
polished in the same order that they were cut. After the last star facet
has been polished, the stone is removed from the dop-stick. Any excess
dop-wax is removed from the stone by means of a solvent, and the full
beauty of the finished gem is revealed.
Tumbled Gems
One other method of finishing gemstones that deserves mention is
tumbling. “Baroque” or “free-form” stones are produced in this manner.
Loose pebbles or pieces of gem materials left over from other cutting
processes are placed in a small barrel or specially constructed box with
loose carborundum grit. The barrel is turned by means of a small motor,
and the abrasion of the pebbles and grit against each other tends to
round the pebbles and give them a finely pitted surface. Progressively
finer and finer carborundum grit is used, and eventually a polishing
compound. The result is several pounds of well-polished gem pebbles of
various shapes and sizes. These baroque stones have found recent favor
in costume jewelry of modern design. The tumbling process is rather
slow, commonly requiring several days or weeks. However, little effort
is involved on the part of the lapidary, and, consequently, the cost of
most tumbled or baroque stones is quite modest. Only gem material that
is unsuitable for cutting in other manners should be finished in this
way.
[Illustration: Fig. 12. Proper placing of the pavilion girdle facets.
Left: facets not joined. Center: facets overlapped, joined too high.
Right: correct placing.]
Stone
Dop-wax
Dop-stick
Chuck
[Illustration: Fig. 13. Proper sequence of cutting of the crown facets.
Left to right: four main facets; all eight main facets; half of the
crown girdle facets; completed crown.]
TEXAS GEMSTONES
Amber
_Composition_: fossil resin. _Crystal system_: amorphous. _Hardness_:
about 2.0 to 2.5. _Specific gravity_: variable, from 1.05 to 1.10.
_Luster_: resinous. _Color_: brown, yellow, red, orange, and white.
_Streak_: white to yellowish to gray. _Cleavage_: none. _Fracture_:
conchoidal. _Tenacity_: brittle. _Diaphaneity_: transparent to
translucent. _Refractive index_: variable, about 1.54. Burns with a
sweet, piney odor.
Rich brown to yellowish amber has been found near Eagle Pass, Maverick
County, in Cretaceous coal and on Terlingua Creek, Brewster County.
Although much of this material is translucent and the quality suitable
for lapidary purposes, the pieces are seldom more than a fraction of an
inch in diameter.
Occasional finds of poor quality brownish amber have been reported from
the Tertiary formations of the Gulf Coastal Plain, but thus far no gem
quality material has been found.
The softness of amber limits its use to brooches, necklaces, and other
jewelry that is relatively safe from abrasion.
Augite
_Composition_: CaMgSi₂O₄; may also contain iron, aluminum, and
sometimes titanium. _Crystal system_: monoclinic. _Hardness_: 5 to 6.
_Specific gravity_: 3.2 to 3.6. _Luster_: vitreous to dull. _Color_:
dark greenish brown and greenish black. _Streak_: light grayish green.
_Cleavage_: two directions, poor. _Fracture_: conchoidal to uneven.
_Tenacity_: brittle. _Diaphaneity_: opaque to translucent. _Refractive
index_: variable, about 1.60 to 1.71.
Augite of gem quality occurs near Eagle Flat, Hudspeth County, Texas.
Although this material is very dark greenish brown and not commonly
thought of as a gemstone, lapidaries have used it to fashion black
faceted stones and cabochons that resemble obsidian. Most of the augite
occurs as loose pieces and crystal fragments that have weathered out of
nearby igneous rocks; the augite can also be found in situ in the
igneous rocks.
Specimens and pieces of cutting quality 1 inch in diameter are common,
and fragments over 2 inches in diameter have been found. The augite is
associated with black spinel and some dark gray to black pieces of
natural glass. Although the faceted and cabochon-cut stones are not
particularly attractive, some of the larger pieces of augite might be
utilized for carving.
Beryl
_Composition_: Be₃Al₂(SiO)₆. _Crystal system_: hexagonal. _Hardness_:
7.5 to 8.0. _Specific gravity_: 2.63 to 2.80. _Luster_: vitreous.
_Color_: pale blue, blue, green, yellow, brownish, pink, and
colorless. _Streak_: white. _Cleavage_: one direction, very imperfect.
_Fracture_: conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_:
transparent to subtranslucent. _Refractive index_: 1.56 to 1.60.
_Dispersion_: low.
Gem-quality beryl has not been reported in Texas. A discussion of beryl
is included herein because the writer believes it likely that beryl of
gem quality will be found in Texas as a result of future investigations
and exploration.
Beryl crystals have been found in pegmatite dikes in Llano, Blanco, and
Gillespie counties. These crystals are commonly several inches long and
exceed 1 inch in diameter but are very badly fractured. Most of the
beryl crystals do not approach gem quality and are entirely unsuitable
for any lapidary use. The color of the crystals found thus far is
bluish, greenish, pinkish brown, yellowish, and colorless. Some very
tiny colorless beryl crystals have been found that are transparent, but
thus far such crystals have been too small to be cut into gems.
Fine blue beryl crystals have been found in the Franklin Mountains near
El Paso, Texas. Unfortunately, these crystals are so badly flawed and
fractured that they are not suitable for lapidary use.
It seems likely that careful prospecting of Texas pegmatites will reveal
at least some gem-quality beryl.
Celestite
_Composition_: SrSO₄. _Crystal system_: orthorhombic. _Hardness_: 3.0
to 3.5. _Specific gravity_: 3.95 to 3.98. _Luster_: vitreous. _Color_:
white, blue, greenish, reddish, and brownish. _Streak_: white.
_Cleavage_: three directions, although one of these directions is not
easily developed. _Fracture_: uneven. _Tenacity_: brittle.
_Diaphaneity_: transparent to subtranslucent. _Refractive index_: 1.62
to 1.63. _Dispersion_: moderate.
Celestite is very seldom cut into gems. Being very soft, brittle, and
having three cleavages, celestite is completely unsuitable for jewelry.
These same properties make this mineral exceedingly difficult to facet;
however, faceted stones are seen in large collections.
[Illustration: Fig. 14. Common crystal form of Travis County celestite.
Same crystal form as shown in Plate I, A.]
Fine crystals of colorless and blue gem-quality celestite (Pl. I, A, and
fig. 14) have been found at Mount Bonnell and other localities west of
Austin, Travis County. The celestite crystals occur in vugs or geodes in
limestone. The crystals are mostly white or colorless and fractured near
the base or where attached, but the tips of the crystals are commonly
clear celestine blue and completely free of flaws.
Crystals several inches in length have been found, but the average size
is about 1 inch. The smaller crystals are frequently more transparent
and consequently better suited for cutting. It is very difficult to
obtain crystals that will allow the cutting of flawless stones of more
than 4 or 5 carats.
Bluish and colorless celestite of gem quality and fine crystals have
been found near Lampasas, Lampasas County, and near Brownwood, Brown
County, but neither of these localities has been very productive of good
gem material.
Celestite geodes have been found in parts of Coke, Fisher, and Nolan
counties, but these geodes contain little gem material.
Diamond
_Composition_: carbon. _Crystal system_: isometric. _Hardness_: 10.
_Specific gravity_: 3.51 to 3.53. _Luster_: adamantine to greasy.
_Color_: brown, colorless, pink, blue, yellow, and various other light
colors; rarely deeply colored; sometimes black. _Cleavage_: four
directions, octahedral, perfect. _Fracture_: conchoidal. _Tenacity_:
brittle. _Diaphaneity_: transparent to opaque. _Refractive index_:
2.42. _Dispersion_: high.
There is only one well-authenticated find of diamond in Texas. A small
brownish diamond was found in 1911 on section 64, block 44, Foard County
(Sterrett, 1912, pp. 1040-1041). The exact weight of the stone has not
been recorded, but one authority estimated that it was of sufficient
size and clarity to yield a cut stone of about one-quarter carat.
The only diamond-bearing rocks known in the United States are in Pike
County, Arkansas. Although many other diamonds have been found in the
United States, all were loose in gravels or streams except for some
stones at the Arkansas locality. The fact that one diamond was found in
Foard County does not mean that the prospects of finding more diamonds
in Texas are much better there than anywhere else in the State. It is
highly unlikely that more than a very few diamonds will ever be found in
Texas, and any stones that may be found in the future are likely to be
widely scattered.
Epidote
_Composition_: HCa₂(Al, Fe)₂Si₃O₁₃. _Crystal system_: monoclinic.
_Hardness_: 6 to 7. _Specific gravity_: 3.25 to 3.5. _Luster_:
vitreous. _Color_: yellowish green to brownish green and brown.
_Streak_: uncolored to grayish. _Cleavage_: two directions.
_Fracture_: uneven. _Tenacity_: brittle. _Diaphaneity_: transparent to
opaque. _Refractive index_: about 1.72 to 1.77.
Llano County has furnished some green and brownish-green epidote that is
suitable for cutting into cabochons. Most of the material that
approaches gem quality has come from contact metamorphic zones and is
associated with garnet, quartz, and some scheelite. Some small cavities
in the rocks contain tiny transparent crystals of gem quality, but the
largest obtainable flawless faceted stones would probably be less than
15 points.
Faceted stones of epidote are sometimes known as pistacite owing to
their common pistachio-green color.
Fluorite
_Composition_: CaF₂. _Crystal system_: isometric. _Hardness_: 4.
_Specific gravity_: 3.0 to 3.25. _Luster_: vitreous. _Color_: violet,
blue, colorless, green, yellow, brown, rose, and crimson red.
_Streak_: white. _Cleavage_: four directions, octahedral, perfect.
_Fracture_: subconchoidal to splintery. _Tenacity_: brittle.
_Diaphaneity_: transparent to subtranslucent. _Refractive index_:
1.434.
Very fine green, transparent fluorite has been found near Voca, Mason
County. The fluorite occurs as vug fillings in pegmatites, associated
with crystals of pink microcline and colorless quartz. Most of the vugs
have been completely filled by the fluorite; therefore, crystals (fig.
15) of the fluorite are not too common. Masses of fluorite several
pounds in weight, rich green, and quite transparent have been found near
Voca. Transparent pieces an inch or more in diameter are common.
[Illustration: Fig. 15. Common crystal form of fluorite.]
Fluorite is much too soft for everyday use in jewelry and because of the
low refractive index does not yield brilliant faceted stones. The
perfect four-directional cleavage, relative softness, and brittle
tenacity of the mineral make it difficult to facet. Faceted stones are
seldom seen outside of collections. Cabochons are also difficult to cut
from this material, but the rich color obtained is ample reward for the
time and care necessary in cutting.
Fluorite occurs at several other localities in Texas, notably in
Hudspeth, Brewster, Presidio, Llano, and Burnet counties, but not
commonly in gem quality or colors that warrant its use as gem material.
Fossil Wood
Wood that is buried in silica-rich sediments is commonly replaced by
quartz, agate, or opal. The wood structure, including a large number of
the annular rings, knots, small branches, and bark, may be preserved.
This process of replacement by silica is believed to take considerable
time. Preservations by other means (_see_ Jet, p. 22) are known, but
silica replacements are most commonly used as gem materials.
Fossil wood is often used by lapidaries as gem material when mineral
replacement preserves the wood structure sufficiently well and when
various impurities color the replacement material attractively.
Excellent gem-quality fossil wood (Pl. I, B) has been found at a great
number of localities in Texas. Agatized and opalized wood occurs in
great abundance along the outcrops of Eocene and Oligocene strata of the
Texas Gulf Coastal Plain. Much of this material is very well suited for
cabochons, bookends, and other lapidary uses. The preservation is
especially good at numerous localities in Washington, Lee, Fayette, and
Gonzales counties, and the variety of colors, such as bluish, gray,
brown, red, yellow, and black, makes this material especially sought
after by “rock-hounds.” Some of the agatized and opalized wood
fluoresces yellow or green under ultra-violet light. The fossil wood is
sometimes found as stumps, limb sections, or large trunk fragments, but
the great majority of the gem material is found as small broken
fragments or stream-rolled cobbles.
Fossil palm wood is by far the most sought after variety because this
material displays “eyes” and tube-like structures that yield very
attractive cabochons and cabinet specimens. Texas fossil palm wood is
highly regarded by cutters from all parts of the country, and this
material is thought by many lapidaries to be some of the finest
gem-quality fossil wood in the United States.
Gravel pits and river gravels in Live Oak County have produced very fine
agatized wood. Although the gem material does not seem to be as abundant
in this area as it is in counties to the northeast, the vivid colors and
excellent preservation of the fossil wood in Live Oak County have
attracted collectors from all over the State. The fossil wood usually
occurs as large rounded cobbles in the streams. Much of this material is
quite translucent when cut and contains various shades of brown, orange,
and red.
The gravels of the Rio Grande have produced some fossil wood in addition
to the excellent agate that is also found there. Most of the fossil wood
found in these gravels is very well preserved, but the colors are
commonly dull shades of brown. Occasional fine red and yellow specimens
have been recovered from the Rio Grande gravels, but these are rare.
Good agatized wood has been found in and near Palo Duro Canyon,
Armstrong County, about 50 miles southeast of Amarillo. Large trunk
sections are not uncommon, but most of the material of cutting quality
is obtained from small fragments. The Palo Duro Canyon fossil wood
greatly resembles the famous Arizona Petrified Forest wood but is not
nearly as plentiful. The Palo Duro wood contains yellow, brown, red, and
bluish colors most commonly. Some of the wood-producing area is within
Palo Duro Canyon State Park which is, of course, closed to collecting.
The surrounding area has been worked diligently by local collectors, but
new pieces of wood are exposed after heavy rains.
Webb and Duval counties have also produced some good fossil wood
specimens.
Gadolinite
_Composition_: Be₂FeY₂Si₂O₁₀. (Various other rare-earth elements may
substitute into this mineral structure.) _Crystal system_: monoclinic.
_Hardness_: 6.5 to 7.0. _Specific gravity_: about 4.2. _Luster_:
vitreous to greasy. _Color_: black; in thin splinters dark bottle
green. _Streak_: white to greenish. _Cleavage_: none. _Fracture_:
conchoidal to splintery. _Tenacity_: brittle. _Diaphaneity_: opaque to
subtransparent in thin pieces. _Refractive index_: variable, about
1.77 to 1.82.
Gadolinite as a cut gem is not seen outside of large collections;
however, it can be faceted into black opaque stones of little beauty but
of great interest to collectors. The best known locality of this mineral
in the United States is Baringer Hill, Llano County, Texas.
Unfortunately, this locality was completely flooded by the completion of
Buchanan Dam in 1938. Masses and rough crystals of gadolinite weighing
over 100 pounds were mined from this locality. The gadolinite occurred
in a large, very coarse-grained pegmatite dike associated with quartz,
microcline, and fluorite, as well as allanite, fergusonite, nivenite,
cyrtolite, thorogummite, and various other rare minerals. Some of the
minerals in the dike occurred in very large masses. One quartz mass over
40 feet in diameter was noted, and microcline masses up to 30 feet in
diameter were not uncommon. Much of the gadolinite was used by
industrial firms as a source of thorium compounds, although some
specimen and gem material found its way into museums and private
collections. Because the locality was worked mostly from 1910 to about
1925 and because since 1938 the waters of Lake Buchanan have completely
flooded the entire area, material from this locality is now exceedingly
difficult to obtain. The collection of the Smithsonian Institution,
Washington, D.C., contains a cut and polished gem of Baringer Hill
gadolinite that weighs 8.6 carats. This mineral is radioactive because
of the presence of uranium, thorium, and other rare radioactive
elements.
Garnet
The garnet group of minerals is variable in composition. Listed below
are the pure members of this group, but garnets found in nature are
usually a mixture of two or more of these end members.
Aluminum garnet—
Grossularite (calcium-aluminum garnet), Ca₃Al₂(SiO₄)₃
Pyrope (magnesium-aluminum garnet), Mg₃Al₂(SiO₄)₃
Almandite (iron-aluminum garnet), Fe₃Al₂(SiO₄)₃
Spessartite (manganese-aluminum garnet), Mn₃Al₂(SiO₄)₃
Iron garnet—
Andradite (calcium-iron garnet), Ca₃Fe₂(SiO₄)₃; may contain
magnesium, titanium, and yttrium
Chromium garnet—
Uvarovite (calcium-chromium garnet), Ca₃Cr₂(SiO₄)₃
Since almandite is the only variety of garnet known to occur commonly in
gem quality in Texas, the following properties are for almandite except
where noted.
_Crystal system_: isometric (all varieties). _Hardness_: about 7.5.
_Specific gravity_: 4.25. _Luster_: vitreous to resinous. _Color_:
red, deep red, and brownish red (other varieties also yellow, white,
orange, pink, black, and green). _Streak_: white. _Cleavage_: none.
_Fracture_: subconchoidal to uneven. _Tenacity_: brittle to tough.
_Diaphaneity_: transparent to subtranslucent. _Refractive index_:
about 1.83.
Good crystals of gem-quality almandite garnet have been found in Llano,
Blanco, Burnet, and Gillespie counties. In southeast Llano County,
northwest Blanco County, and northeast Gillespie County, the stones
mostly occur in stream gravels where they have collected after being
weathered out of compact mica schists. Owing to the fact that most of
the garnets have not been transported very far from their source, the
stones commonly show good crystal form (Pl. II, A). All of the garnets
from one locality commonly do not have exactly the same crystal form.
The garnets are mostly widely scattered in the stream gravels but can be
found concentrated behind rocks and on small gravel bars.
Many of the crystals are less than one-eighth inch in diameter; however,
good crystals one-fourth to one-half inch in diameter are common. Most
of the stones are too fractured or have too many inclusions to yield
gems, but many transparent stones have been found. The transparent
crystals usually yield flawless deep red faceted stones of 2 carats or
less. Some of the stones that contain too many inclusions to facet are
cut as cabochons and are then often known as carbuncle.
Small garnet fragments have been found in streams and in gneisses and
pegmatites near Castell, Llano County, but they are not commonly of gem
quality.
Occasional small gem-quality garnets have been found in pegmatites and
contact metamorphic zones in Burnet County. Garnets have also been found
in several other counties, notably Mason, El Paso, Hudspeth, and
Culberson, but no stones of facet quality have been reported.
Jet
_Composition_: a variety of brown coal or lignite. _Structure_: woody.
_Hardness_: 3 to 4. _Specific gravity_: about 1.30 to 1.35. _Luster_:
dull. _Color_: black, brownish black. _Streak_: brown to brownish
black. _Cleavage_: none. _Fracture_: uneven to smooth. _Tenacity_:
tough to slightly brittle. _Diaphaneity_: opaque. Burns with a sooty
yellowish flame.
Jet is a type of fossil wood in which there has been sufficient chemical
change to make the wood relatively hard and black without destroying the
woody structure. The best specimens of jet polish into lustrous black
cabochons.
Jet occurs in Presidio County as compressed and flattened trunks of
trees in a thin layer of coal and lignite in Cretaceous strata 100 to
200 feet stratigraphically below the San Carlos beds.
Specimens of “jet” have been found in some of the lignitic Tertiary
strata of the Texas Gulf Coastal Plain; however, this material is mostly
soft, brownish, and not of gem quality.
Labradorite
_Composition_: NaAlSi₃O₈, 50% to 30%; CaAl₂Si₂O₈, 50% to 70%. _Crystal
system_: triclinic. _Hardness_: 6.0 to 6.5. _Specific gravity_: about
2.60. _Luster_: vitreous to sometimes pearly. _Color_: straw yellow,
white, greenish, gray, reddish, bluish, and green. Sometimes shows a
play of colors on particular cleavage surfaces. _Streak_: uncolored.
_Cleavage_: three directions. _Fracture_: uneven to conchoidal.
_Tenacity_: brittle. _Diaphaneity_: transparent to translucent.
_Refractive index_: about 1.56. _Dispersion_: low.
Very fine facet-quality labradorite has been found about 20 miles south
of Alpine, Brewster County. The labradorite occurs loose in the soil as
slightly weathered or frosted cleavage fragments, commonly showing one
or more crystal faces (Pl. II, B). The pale-yellow or straw-yellow color
of these fragments, as well as their lack of internal imperfections,
makes these stones excellent gem material. Individual pieces that exceed
three-fourths inch in their longest dimensions are rare. Cut stones of
more than 5 or 6 carats from this locality are scarce. The source of
this material is uncertain, but it is probably weathering out of an
underlying igneous rock.
Microcline
_Composition_: KAlSi₃O₈. _Crystal system_: triclinic. _Hardness_: 6.0
to 6.5. _Specific gravity_: 2.54 to 2.57. _Luster_: vitreous to
pearly. _Color_: white, pale yellow, red, blue green, bluish.
_Streak_: white. _Cleavage_: four directions, usually three of these
distinct. _Fracture_: uneven. _Tenacity_: brittle _Diaphaneity_:
transparent to translucent. _Refractive index_: about 1.52 to 1.53.
Very fine crystals of blue microcline have been found east of Packsaddle
Mountain and near Kingsland in Llano County. Crystals exceeding 1 foot
in length have been found, although most are only a few inches long. The
color of the microcline is mostly pale blue, but some crystals are
darker. Microcline crystals associated with milky or vein quartz, smoky
quartz, some biotite, and rarely cassiterite occur in pegmatite dikes
which vary in size from a few inches to several feet in thickness. The
color of this microcline is pale in comparison to microcline from some
other localities in the United States, but the Texas blue microcline
does yield pleasing cabochons. Perfect crystals of this material are
prized by collectors. Blue or greenish microcline is often called
amazonite or amazon stone.
Bluish microcline associated with quartz and topaz has also been
reported near Katemcy, Mason County.
Red microcline is common in several central Texas counties and is a
primary constituent of many of the igneous rocks in those counties.
Large crystals of perthitic red microcline occur in pegmatite dikes of
Mason, Llano, Burnet, and Gillespie counties. Any feldspar quarry or
other pegmatite mining operation in any of these counties is likely to
contain large red microcline crystals and fragments. Unfortunately, the
good crystals that may have been present are often shattered by blasting
during quarrying operations.
Feldspar quarries in northeastern Gillespie County have yielded some
good red cabochon material as well as good crystals. Here the microcline
occurs with milky and smoky vein quartz, smoky quartz crystals, clear
quartz crystals, greenish muscovite, and biotite. Many of the older
quarries in Gillespie County have not been active for some time, and the
dumps and quarry walls have been diligently searched by collectors.
[Illustration: Fig. 16. Crystal faces on microcline specimen shown in
Plate III, A.]
Many of the pegmatite dikes near Lake Buchanan in Llano and Burnet
counties have produced some good red microcline specimens and cutting
material (Pl. III, A, and fig. 16). Many of these crystals are more
pinkish than those in Gillespie County, but this is commonly due to the
fact that the crystal faces of the Lake Buchanan area crystals are
somewhat more weathered than the fresh Gillespie County crystals.
Numerous other local areas in the counties mentioned, as well as some
localities in Hudspeth and Culberson counties, have also produced small
amounts of red and pink microcline of gem quality.
Obsidian
_Composition_: volcanic glass. _Structure_: amorphous. _Hardness_: 5.0
to 5.5. _Specific gravity_: 2.3 to 2.5. _Luster_: vitreous. _Color_:
black, dark gray, reddish, brown, bluish, and greenish. _Streak_:
white. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: brittle.
_Diaphaneity_: translucent to nearly opaque. _Refractive index_:
variable, about 1.45 to 1.53.
Gem-quality black and dark-gray obsidian has been found in Presidio
County associated with extrusive igneous rocks. The obsidian in this
area is too opaque to serve as attractive faceted stones but is found in
pieces of sufficient size and quality to yield nice cabochons. Some of
the small weathered pieces of this material resemble tektite in outward
appearance; in fact, the “valverdites” mistaken originally for tektites
are pebbles of weathered obsidian in terrace gravel of Val Verde County.
Obsidian takes a high polish but is very sensitive to heat. Stones that
are slightly overheated during grinding or sanding will quickly shatter.
Obsidian of gem quality has been reported also in Brewster County.
Opal
_Composition_: SiO₂·nH₂O. _Structure_: amorphous. _Hardness_: 5.5 to
6.5. _Specific gravity_: 1.9 to 2.3. _Luster_: subvitreous to pearly.
_Color_: white, bluish, pink, brown, yellow, and gray. _Streak_:
white. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: brittle.
_Diaphaneity_: transparent to nearly opaque. _Refractive index_: 1.43.
Opal other than as fossil or opalized wood (pp. 20-21) occurs at the
following several localities in Texas.
Approximately 16 miles south of Alpine, Brewster County, precious opal
occurs in very small seams and as cavity fillings in very hard
pinkish-brown rhyolite. This opal is milky or bluish and commonly
exhibits small flashes of blue, green, red, and orange fire. Individual
pieces of this opal are mostly quite small, rarely over one-fourth inch
in diameter, and very difficult to remove from the tough rhyolite
matrix. Local lapidaries have cut interesting cabochons from this
material in which several small patches of opal that are close together
in the matrix are included in the same cabochon.
Small finds of opal associated with rhyolites and basalts have come from
other localities in west Texas, but the opal mostly does not display
enough play of colors to warrant its use as gem material.
Near Freer, Duval County, some very attractive common opal has been
found. The opal is colored various shades of pink, blue, and yellow and
in certain local areas occurs as fragments that are cemented together by
clear chalcedony. Various colors are commonly found in the same piece,
and such material yields handsome cabochons. Although the area has never
been worked commercially, it has been hunted by collectors and cutters
for several years.
Pearl
Pearls are the result of the secretion of calcium carbonate by various
shellfish around sand grains, parasitic organisms, shell fragments, or
other foreign objects that have in some way entered the body cavity of
the shellfish. Since the shellfish is unable to expel these irritating
particles or organisms, it deposits successive layers of calcium
carbonate around the foreign substance to make it smoother and less
irritating. Although pearls are principally calcium carbonate, they also
contain small amounts of an organic substance, called conchiolin, and
water. Pearls are found in shellfish that live in either fresh or salt
water. Few pearls are spherical in shape; most are rounded but somewhat
irregular and are known as baroque pearls. Good quality pearls are the
only gemstone commonly sold by the grain, a unit of weight equal to 0.25
carat or 0.05 gram. The pearl grain is not the same unit of weight as
the Troy grain.
In Texas, pearls have been found in fresh-water clams in most of the
major rivers and streams, notably in the Brazos, Concho, Colorado,
Guadalupe, Llano, Nueces, Sabine, Rio Grande, and Trinity Rivers.
Several Texas lakes have also yielded pearls, notably Caddo Lake and
other lakes in north-central and northeast Texas.
Small pearls are frequently found along the Texas Gulf Coast in edible
oysters and other common shellfish. Fossil pearls have also been found
but because of their darkened appearance are of value only as
curiosities.
The pearls thus far found in Texas have been of relatively poor quality
and show little or no iridescence. These pearls have little value except
as curiosities, although one writer has stated that the discovery of
pearls in the Nueces River led to the original Spanish settlement of the
State (Baker, 1935, p. 569).
Quartz
_Composition_: SiO₂. _Crystal system_: hexagonal. _Hardness_: 7.
_Specific gravity_: 2.65 to 2.66 in crystals. _Luster_: vitreous, also
waxy, greasy, and dull. _Color_: most often colorless, brown, yellow,
violet; sometimes green, red, blue, and black; cryptocrystalline
varieties often variously colored by impurities. _Streak_: white.
_Cleavage_: indistinct. _Fracture_: conchoidal to splintery.
_Tenacity_: brittle to tough. _Diaphaneity_: transparent to opaque.
_Refractive index_: 1.544 to 1.553.
The quartz family gemstones can be divided into two groups for purposes
of description. The first group is the crystalline varieties, or those
quartz varieties that commonly occur in distinct crystals. The second
group is the cryptocrystalline varieties, or those quartz varieties that
occur as irregular masses that are composed of many microscopic
crystals. The crystalline varieties are usually much more transparent
and are most often seen as faceted stones. The cryptocrystalline
varieties vary from subtransparent to opaque and are almost always cut
as cabochons.
CRYSTALLINE VARIETIES
_Amethyst_ (violet to purple-colored quartz).—A northeastern Gillespie
County locality known as Amethyst Hill has produced quite a number of
fine light to medium violet amethyst crystals which occur in quartz
veins and geodes associated with serpentine and talc. Many crystals have
been found loose in the soil.
The amethyst tends to be very irregularly colored in zones parallel to
the crystal faces. In many, the base of the crystal is colorless or
white and only the termination is violet. Crystals up to 3 inches long
have been found at this locality, but the average size is much less.
The surface at this locality is almost entirely depleted of amethyst,
with only an occasional small crystal or fragment to be seen. However,
small excavations are still sometimes productive.
Good groups of pale amethyst crystals have been found in quartz veins
near the old town site of Oxford, Llano County. The occurrence seems to
be much the same as the Amethyst Hill locality. Little exploration for
gemstones has been done in this area, and future discoveries seem
likely.
Chalcedony geodes lined with amethyst crystals have been found in
Brewster, Presidio, Culberson, and Hudspeth counties, but the
occurrences are scattered. The crystals are seldom large enough to yield
gems of more than 3 carats and are mostly very light colored.
A few pieces of gem-quality amethyst have been found in Burnet County.
_Citrine_ (yellow quartz).—Very little gem-quality citrine has been
reported in Texas. Some small citrine crystals have been found at
Amethyst Hill in northeastern Gillespie County, but few are of
sufficient size or color to yield good gems.
The writer has seen one citrine crystal that was found in the gravels of
a small stream in eastern Llano County near Buchanan Dam. The crystal
weighs about 1 ounce and is perfectly clear, light golden yellow, and
flawless. However, a further search of the stream gravels failed to
produce any other citrines.
_Rock crystal_ (colorless quartz).—Numerous localities in Texas produce
this colorless variety of quartz, which is the most common variety of
facet quality quartz and consequently is of little value.
Rock crystal occurs at many localities in Burnet, Llano, and Mason
counties. The crystals mostly occur in pegmatite dikes or in stream
gravels where they have been weathered out of their parent rock. Some
fine colorless quartz crystals have been found near Voca, Mason County,
in weathered pegmatite dikes and also loose in the sands of nearby
streams. Crystals from this locality are often stained with reddish iron
oxide on their outer surfaces. Some of the rock crystal found near
Katemcy, Mason County, shows asterism when cut with the proper
orientation. Fine clear colorless crystals up to 8 inches long have been
found in the pegmatite dikes near Lake Buchanan in both Llano and Burnet
counties. Several localities near Enchanted Rock in Llano County have
also produced some good colorless crystals.
Feldspar quarries in large pegmatites in northeastern Gillespie County
have yielded attractive quartz crystals, some of which contain smoky
phantom crystals and tourmaline inclusions.
Some pieces of rock crystal enclosing green, needle-like actinolite
crystals have been found near the Llano-Gillespie-Blanco County corner.
This material is not suitable for faceted gems but does lend itself to
interesting and attractive cabochons.
Colorless quartz crystals commonly are found lining small chalcedony
geodes in Brewster, Presidio, Culberson, Hudspeth, Reeves, and Jeff
Davis counties. These crystals are most commonly less than 1 inch long
but are mostly very clear.
Rock crystal has been found in crevices of petrified wood in many east
and southeast Texas counties, although the crystals are mostly quite
small.
Many lesser occurrences of rock crystal, too numerous to mention, are
located within the State.
_Rose quartz_ (pink quartz).—Rose quartz occurs at various localities in
Burnet, Llano, Mason, and Gillespie counties, but the amount of material
is mostly small and the greater part unsuitable for gem purposes. Some
good pink rose quartz occurs near Town Mountain, Llano County, but this
material does not have flawless areas large enough to yield faceted
stones of more than a few carats. Rose quartz is always slightly milky,
or cloudy, and does not cut into brilliant faceted stones. The Town
Mountain rose quartz has been cut into attractive cabochons.
_Smoky quartz_ (brown, yellow-brown, and golden-brown quartz).—Several
Texas localities have produced fine smoky quartz. Baringer Hill, a noted
rare-earth minerals pegmatite locality in Llano County, contained some
smoky quartz crystals that were estimated to weigh over 1,000 pounds,
and the locality produced many smaller crystals that were of gem
quality. Baringer Hill was flooded by the completion of Buchanan Dam in
1938 and is presently under the waters of Lake Buchanan. A few fine
golden-brown gem-quality crystals have been found along the lake shore
and in small pegmatites nearby (Pl. III, B.).
Feldspar quarries in northeastern Gillespie County have produced smoky
quartz crystals that exceed 1 foot in length, but these crystals are
mostly flawed, possibly as a result of blasting, and mostly contain only
small clear areas.
Good color smoky quartz crystals are found with topaz in the pegmatites
and stream beds in Mason County, near Streeter, Grit, and Katemcy. These
crystals tend to be lighter colored than those near Lake Buchanan, but
they commonly contain large flawless areas.
CRYPTOCRYSTALLINE VARIETIES
_Chalcedony._—When free from impurities of various oxides and other
compounds, chalcedony has little to render it pleasing as a gemstone. It
is mostly gray, white, brown, or bluish and commonly has a waxy luster.
Some of the chalcedony found along the Rio Grande Valley and in west
Texas will take dyes, and local lapidaries have had some success in
dyeing this material various shades of blue, green, yellow, and red.
When the chalcedony is naturally colored and variegated, usually in
bands, mossy figures, or dendritic forms, it is called agate.
_Agate_ (variegated chalcedony).—The wide variety of markings and colors
available together with the ease of cutting make agate a favorite of
many lapidaries. Fine agate has been found at numerous localities in
west and south Texas. Fine plume agate, famous throughout the United
States, is found south of Alpine. Plume agate is characterized by
dendritic or tree-like inclusions and is mostly cut into very handsome
cabochons. The agate from south of Alpine commonly contains black, red,
yellow, or brown plumes within the same piece. The variety of colors and
lack of porosity of this agate make it highly desired among lapidaries.
The agate occurs loose on the surface of the ground and in the soil in
small nodules that have a very rough, brownish surface. These nodules
are mostly less than 3 inches in diameter, although specimens of gem
quality have been found that exceed 200 pounds.
Some very fine agate has been found in the vicinity of Needle Peak,
Presidio County. This material is mostly green moss agate in clear
chalcedony and commonly contains small yellow “sun-burst” figures. The
contrasting yellow and green design makes very beautiful cabochons.
Fine agate has been found south of Marfa, Presidio County. This agate is
mostly clear chalcedony with black, yellow, or variously colored plumes,
moss, or “bouquet-like” figures.
Numerous other localities in Presidio and Brewster counties have
produced good agate.
Various amounts of agate, jasper, and chalcedony occur in the gravels of
the Rio Grande in varying quantities from Big Bend National Park
downstream to Brownsville. This agate is found both in the present river
gravels and in the older river gravels that now are located on nearby
hills and slopes up to several miles north or south of the present Rio
Grande. The greatest concentration of agate and related gem materials
seems to be in the area between Laredo and Rio Grande City. Vast
quantities of excellent gem material have been removed from this area
for many years (Pl. IV). The agate occurs as rounded, stream-worn
cobbles and commonly has a thin white coating that makes it difficult to
distinguish from the abundant chert and other rocks. The agate occurs in
cobbles that are mostly 3 to 6 inches in diameter, but specimens of gem
quality that exceed twice this size are known. The agate varies greatly
in design and color. Plume, moss, banded, and sagenitic agate occur in
these gravels in a wide variety of colors. The jasper in the Rio Grande
gravels is yellow, red, green, or various shades of these and is
commonly suspended as angular fragments in clear chalcedony.
Good agate has also been found near Balmorhea in Reeves and Jeff Davis
counties and in smaller amounts at numerous other west and south Texas
localities.
_Agatized wood_ (_see_ Fossil wood, pp. 20-21).
_Carnelian_ (translucent reddish chalcedony).—This variety of chalcedony
in small quantities has been reported from near Van Horn, Hudspeth
County. Small pieces of carnelian have been found in the gravels of the
Rio Grande, but finds have been few and scattered.
_Jasper_ (impure opaque or subtranslucent quartz).—Good green, yellow,
red, and brown jasper has been found in the gravels of the Rio Grande at
all of the localities that produce agate. The colors are quite vivid,
and the material takes a fine polish. Some pieces of orbicular jasper
(jasper with circular or eye-like markings) have been found in this
material. These gravels commonly contain jasper as fragments that are
suspended in clear chalcedony; this is called brecciated jasper and
yields very handsome cabochons.
Many of the west Texas agate localities also produce jasper in quantity.
Good jasper has been reported from north of Brackettville, Kinney
County. Jasper is a minor constituent of the stream gravels in many
parts of the State.
Sanidine
_Composition_: KAlSi₃O₈; commonly contains some sodium. _Crystal
system_: monoclinic. _Hardness_: 6. _Specific gravity_: 2.57 to 2.58.
_Luster_: vitreous to pearly. _Color_: colorless, white, pale yellow,
and gray. _Streak_: uncolored. _Cleavage_: three directions.
_Fracture_: conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_:
transparent to subtranslucent. _Refractive index_: 1.52 to 1.53.
Some feldspars, including sanidine, show a nice blue sheen in reflected
light parallel to certain crystallographic directions. Stones having
this property are called moonstone. A clear yellowish sanidine showing
an attractive blue sheen has been found in Brewster, Jeff Davis, and
Presidio counties. The individual pieces are small, the average size
being about one-eighth inch. The sanidine is found loose in the soil at
some localities where it has weathered out of rhyolite, and specimens of
the sanidine in the parent rock are not difficult to obtain. Very small
cabochons can be cut from this material, but few lapidaries have done so
because inexpensive larger pieces of moonstone can be obtained easily
from foreign sources. However, the west Texas sanidine does show a blue
sheen when cut and polished.
Spinel
_Composition_: MgAl₂O₄ (magnesium may be replaced in part by ferrous
iron or manganese and the aluminum by ferric iron and chromium).
_Crystal system_: isometric. _Hardness_: 8. _Specific gravity_: 3.5 to
4.1. _Luster_: vitreous to sub-metallic. _Color_: black, pink, red,
blue, green, yellow, brown, and violet. _Streak_: white. _Cleavage_:
one direction, imperfect. _Fracture_: conchoidal. _Tenacity_: brittle.
_Diaphaneity_: transparent to opaque. _Refractive index_: variable,
approximately 1.72 to 2.00.
In many areas of the world, fine quality, beautifully colored,
transparent spinels are found and used as gems. The only gem-quality
spinel reported thus far in Texas is black and opaque. Near Eagle Flat
in Hudspeth County, black spinel crystals have been found associated
with augite and natural glass; these minerals are weathering out of an
intrusive igneous rock. The spinel crystals have an octahedral form
which is common for this mineral (fig. 17). Most of the spinels are free
of flaws, but because of their black color they have little value as
gems. The crystals are found loose in the sand of streams near the
outcrops of the igneous rock or embedded in the rock. They seldom exceed
half an inch in diameter. These stones are primarily sought by
collectors.
[Illustration: Fig. 17. Common crystal form of spinel.]
Tektite (Bediasite)
_Composition_: A natural glass, approximately 75% SiO₂, 15% Al₂O₃, 4%
FeO, also MgO, Na₂O, K₂O, and traces of other elements. _Crystal
structure_: amorphous. _Hardness_: 5 to 6. _Specific gravity_: 2.33 to
2.44. _Luster_: vitreous, often dull on weathered surfaces. _Color_:
dark brown, greenish brown, appears black in thick sections. _Streak_:
uncolored. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_:
brittle. _Diaphaneity_: transparent to subtransparent. _Refractive
index_: 1.488 to 1.512.
The average bediasite size is about 1 inch in diameter, although
specimens approximately 3 inches in diameter are known. The uncut
tektites are very interesting, showing a variety of shapes and surface
features (Pl. V, A) and many exhibit contorted flow structure. The
surface of many tektites is grooved or furrowed, while on others it is
smooth or frosted. The Texas tektites are known as “bediasites,” after
place names in Grimes County traceable to the Bedias Indians who
formerly lived there.
Dark brown and greenish-brown tektites have been found in Texas in
gravels at scattered localities in Walker, Grimes, Brazos, Burleson,
Lee, Fayette, Gonzales, Lavaca, and DeWitt counties. Outside of Texas
the only other authenticated tektite localities in the United States at
the present time are in Dodge and Irwin counties, Georgia. A fragment of
a similar tektite has recently been reported from near Martha’s
Vineyard, Massachusetts. The tektites reported from Oklahoma are now
known to be pebbles of obsidian.
Although tektites have little value or beauty as gemstones, they have
been cut by lapidaries as both faceted and cabochon stones. Tektites
take a high polish but are mostly so dark in color that they appear
black.
The origin of tektites is of great scientific interest and is currently
the subject of much debate. Some scientists believe that tektites are of
meteoritic origin, while others believe that tektites were formed by
various terrestrial processes. Since no one has actually observed a
tektite to fall or form, and many of the theories of origin are
difficult to prove without direct observation, the origin of tektites is
likely to remain in controversy for some time.
Topaz
_Composition_: Al₂(F, OH)₂SiO₄. _Crystal system_: orthorhombic.
_Hardness_: 8. _Specific gravity_: 3.4 to 3.6. _Luster_: vitreous.
_Color_: pale blue, sky blue, greenish, white, wine yellow, straw
yellow, grayish, pink, reddish, and orange. _Streak_: uncolored.
_Cleavage_: one direction, basal, highly perfect. _Fracture_:
conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: transparent
to subtranslucent. _Refractive index_: about 1.60 to 1.63.
_Dispersion_: moderate.
Various yellow and smoky colored quartz gems are offered for sale as
“Spanish Topaz,” “Smoky Topaz,” “Madeira Topaz,” and “Topaz Quartz.”
These names are entirely misleading and should be dropped from usage.
Fine gem-quality white, pale-blue, and sky-blue topaz has been found
near Streeter, Grit, and Katemcy, Mason County. This Texas gem material
compares favorably in color, size, and clarity with topaz found anywhere
in the United States. Fine crystals of topaz (Pl. V, B, and fig. 18)
occasionally are found in pegmatite dikes associated with quartz, black
tourmaline, cassiterite, and pink microcline. Many of the gem-bearing
pegmatites have been eroded away, leaving the topaz concentrated in the
stream beds. The stones mostly occur as frosted, stream-worn pebbles
(Pl. VI, A) in the numerous small creeks in the area. The topaz is
heavier than the quartz and microcline that compose the stream gravel
and is commonly found immediately on top of the granite bed-rock in the
bottom of the stream bed. The stones tend to lodge behind boulders or
small dikes cutting across the stream.
[Illustration: Fig. 18. Crystal faces on topaz crystal shown in Plate
V, B. This crystal habit is typical of the topaz from Mason County.]
The white or colorless stones are by far the most common, outnumbering
the bluish stones about ten to one. The color of the blue stones tends
to be irregularly distributed in zones parallel to the crystal faces.
Topaz that is colored in this manner should be cut with the best blue
color near the bottom or culet of the gem (fig. 19). If done correctly,
this will give the entire gemstone the desirable blue color.
[Illustration: Fig. 19. Cross section showing the proper orientation of
dark-color zone in a gem cut from an irregularly colored stone.]
COLORLESS
BLUE
The colorless stones can be turned pale yellow, yellowish brown, or
straw yellow by exposure to X-ray radiation, and some of the bluish
stones will fluoresce faintly yellowish under ultra-violet light.
The largest gem-quality topaz crystal yet found in North America has
come from Mason County. It is a pale-blue crystal weighing 1,296 grams,
now in the collection of the U.S. National Museum. Several other large
pieces, some weighing over a pound, have been found. One large crystal,
exact weight unknown, was found near Katemcy. Several gem cutters have
estimated that this stone could easily yield a single, flawless
pale-blue gem of about 500 carats. Many large gems have been cut from
topaz found in this area, including at least one stone of over 300
carats.
One obstacle in the cutting of topaz is its perfect basal cleavage. The
gemstone should be oriented so that no facet of the stone will be
parallel to or within less than about 5 degrees of the cleavage
direction, or the facet may be very difficult or impossible to polish.
It is difficult to estimate the productivity of this area since its
discovery in the early 1900’s. Few systematic attempts have been made to
exploit the deposits, and a great amount of the topaz thus far recovered
has been found by private collectors. The Mason County topaz deposits
are still very productive, and additional exploration may uncover even
more gem-producing areas.
Topaz has also been found in stream gravels or pegmatites in Burnet,
Llano, Gillespie, and El Paso counties but very rarely in gem quality.
Tourmaline
_Composition_: H₉Al₃(B·OH)₂Si₄O₁₉; hydrogen often replaced by iron,
magnesium, calcium, or fluorine. _Crystal system_: hexagonal.
_Hardness_: 7 to 7.5. _Specific gravity_: 2.98 to 3.20. _Luster_:
vitreous to resinous. _Color_: black, brownish black, brown, blue,
green, red, pink, yellow, and gray. _Streak_: uncolored. _Cleavage_:
two directions, very imperfect. _Fracture_: subconchoidal to uneven.
_Tenacity_: brittle. _Diaphaneity_: transparent to opaque. _Refractive
index_: about 1.62 to 1.64.
Black tourmaline is schorl; brown tourmaline, dravite.
Good crystals of black and dark brown tourmaline occur at Town Mountain
near Llano, Llano County. The tourmaline crystals average about 1 inch
in length, do not commonly exceed 2 inches, and are associated with
white vein quartz. The quartz completely encloses the tourmaline, but
the crystals can be broken free or the quartz can be trimmed away with
the use of a diamond saw. The latter procedure is recommended whenever
possible, for it is very easy to shatter the tourmaline crystals while
trying to remove them from the quartz by other means. Many of the
crystals are completely unsuitable for cutting, being too brittle or too
badly cracked and flawed. However, some small crystals have been found
that are of sufficient quality and size to yield flawless stones of a
few carats. Few of these stones have been cut since the tourmaline is so
dark that it appears opaque, and few persons find a gem of this nature
attractive.
Good black and dark brown crystals of tourmaline associated with
andalusite and graphite occur in the Packsaddle schist (Precambrian)
near Sunrise Beach, Llano County (Pl. VI, B, and fig. 20). Although
generally smaller in diameter than the crystals found at Town Mountain,
they commonly exceed 3 inches in length, although the average size is a
little over 1 inch. Many of these crystals are suitable for cutting into
opaque or nearly opaque stones of about 5 or 6 carats.
Black tourmaline has also been found in Hudspeth and Culberson counties
but not of sufficient quality to be used as a gemstone.
[Illustration: Fig. 20. Common crystal form of Llano County tourmaline.]
Turquoise
_Composition_: hydrous phosphate of aluminum and copper. _Crystal
system_: triclinic. _Hardness_: 5 to 6. _Specific gravity_: variable,
2.6 to about 2.8. _Luster_: dull, sometimes waxy. _Color_: sky blue to
greenish blue. _Streak_: white to greenish. _Cleavage_: none in
massive material, two directions in crystals. _Fracture_: conchoidal
to subconchoidal. _Tenacity_: brittle. _Diaphaneity_: subtranslucent
to opaque. _Refractive index_: 1.61 to 1.65.
Turquoise of good sky-blue to greenish-blue color has been found a few
miles southwest of Van Horn, Culberson County. Several shallow pits were
dug at this locality about 1910; however, the amount of turquoise
produced was small. The main occurrence of the turquoise was in seams
about 1 millimeter thick along joints in the fine-grained rocks of this
area. Persons who have visited Culberson County more recently report
that even minute traces of the turquoise are now difficult to find at
the old prospect pits. However, further prospecting in the area might
yield some additional localities.
Small amounts of turquoise have been reported near El Paso, El Paso
County, and also in volcanic rocks near the Jeff Davis-Brewster County
line, north of Alpine.
A small amount of turquoise has been mined from several localities a few
miles northwest of Sierra Blanca in the Sierra Blanca Mountains of
Hudspeth County.
GLOSSARY
Amorphous—without definite molecular structure; not crystalline.
Baroque stone—an irregularly shaped, polished stone; usually applied
to tumbled stones.
Baroque pearl—an irregularly shaped pearl.
Brilliancy—reflecting much light; having brightness.
Brilliant cut—a mode of arrangement of facets commonly used on round
or oval stones. The standard American brilliant cut has 57 or
58 facets. Most diamonds of 5 or less carats are cut in this
manner.
Cabochon—a stone cut with a flat or convex upper surface; sometimes
faceted in part. Opal, star sapphire, and agate are stones
that are frequently cut in this style (fig. 2).
Cambrian—a division of geologic time, estimated to be the time from
550 to 440 million years ago; the oldest time division of the
Paleozoic era.
Carat—a unit of weight equal to ⅕ of a gram or 0.2 gram. One ounce
avoirdupois is equal to 141.75 carats.
Cleavage—the tendency of certain minerals to split in particular
directions yielding relatively smooth plane surfaces.
Conchiolin—an organic albuminoid substance found in pearls.
Conchoidal—a type of fracture having curved concavities or the
approximate shape of one-half of a bivalve shell. Glass has
excellent conchoidal fracture.
Cretaceous—a division of geologic time, estimated to be the time from
135 to 60 million years ago; youngest division of the Mesozoic
era.
Crown—that portion of a faceted gem above the girdle; the upper
portion of a facet-cut gem (fig. 6).
Cryptocrystalline—composed of very fine or microscopic crystals.
Crystal—the regular polyhedral form, bounded by plane surfaces, that
is assumed by a mineral under suitable conditions. Crystals
have definite external symmetry and internal molecular order.
Crystalline—possessing definite internal molecular order; not
amorphous.
Cubic—in the general shape of a cube. The isometric crystal system is
often called the cubic system.
Culet—the very bottom portion of a faceted gem; the point or line
formed by the intersection of the lowest pavilion facets (fig.
6).
Dendritic—branching or tree-like in form.
Diaphaneity—relative transparency. The diaphaneity of a mineral is
described as transparent, translucent, opaque, etc.
Dike—a tabular rock body, usually igneous in origin, which cuts across
the surrounding rock strata.
Dispersion—a measure of the ability of gemstones to separate complex
or white light into its component colors; often illustrated
with a prism. Gemstones that are capable of separating colors
of light widely are said to have high dispersion; gemstones
not so capable of separating white light into colors are said
to have low dispersion.
Dopping—the act of cementing a gemstone, either rough or partly
finished, to a dop-stick.
Dop-stick—the wooden stick or cylindrical piece of metal to which a
gemstone is cemented to facilitate handling during cutting and
polishing.
Dop-wax—the agent or cement used to secure a gemstone to a dop-stick.
Emerald cut—a rectangular or square faceted stone with beveled corners
whose surfaces are covered with several series of rectangular
facets.
Eocene—a division of geologic time, estimated to be the time from 50
to 40 million years ago; one of the older divisions of the
Cenozoic era.
Extrusive rock—igneous rock that has been extruded or forced out onto
the earth’s surface.
Facet—a single plane polished surface on a faceted gem.
Facet head—a device used in the cutting and polishing of faceted gems;
used to control the placement of facets and their relative
angles (fig. 7).
Facet table—the equipment used in the cutting and polishing of faceted
gems and the table on which most of the equipment is mounted
(fig. 7).
Feldspar—a group of closely related silicate minerals including
orthoclase, microcline, sanidine, plagioclase, labradorite,
and others.
Fire—the reflections of variously colored light from a precious opal;
also the different colors of light reflected from a faceted
gem owing to the dispersion of the mineral.
Fracture—the texture of a freshly broken surface other than a cleavage
surface, described as conchoidal, even, splintery, etc.
Gem—a cut and polished gemstone.
Gemology—the science dealing with the study of gemstones.
Gemstone—a mineral suitable for cutting into a gem; the term gemstones
is frequently used collectively to include both cut and
polished stones and rough stones.
Geode—a rounded or spherical rock cavity; commonly lined with
crystals.
Girdle—the portion of a faceted gem separating the crown from the
pavilion; the girdle may or may not be polished and usually
contains about 2 percent of the total depth of the gem (fig.
6).
Gneiss—a coarse-grained metamorphic rock having segregations of
granular and platy minerals that give it a more or less banded
appearance without well-developed schistosity.
Grain (pearl grain)—a unit of weight equal to 0.05 gram or 0.25 carat;
not the same as the Troy grain.
Granite—a granular igneous rock composed mostly of quartz, feldspar,
and commonly mica and/or hornblende.
Hexagonal—having six angles and six sides; a crystal system in which
the crystal faces are referred to four intersecting axes;
three of these axes are equal, lie in the same plane, and
intersect at angles of 60 degrees; the fourth axis is
perpendicular to the other three.
Igneous rock—rock formed by solidification from a hot melt.
Index of refraction—a measure of the relative ability of a gemstone to
“bend” incident light rays; sine of the angle of incidence of
a light ray divided by the sine of the angle of refraction.
Intrusive rock—rock that has been pushed (usually in a molten state)
among pre-existing rock strata, commonly along faults or
fissures. Intrusive rocks do not reach the earth’s surface but
are commonly exposed at the surface by later erosion.
Isometric—a crystal system in which the crystal faces are referred to
three equal intersecting axes at right angles to each other.
Lap—a disc-shaped piece of metal or other material which is
impregnated with diamond dust, or some other cutting or
polishing agent, that is revolved while the gemstone is worked
against it.
Lap plate—a metal plate to which a cutting or polishing lap is
attached, usually by means of a threaded bolt and wing nut.
The lap plate is attached to the shaft which is turned by the
motor under the facet table.
Lapidary—one who practices the lapidary arts; a gem cutter.
Limestone—a sedimentary rock composed mostly of calcium carbonate.
Luster—the appearance of the freshly broken or unweathered surface of
a mineral in reflected light (p. 5).
Main facet—as applied to the standard American brilliant cut, one of
the first eight facets cut on either the crown or pavilion of
a gem (fig. 6).
Matrix—the material in which a specific mineral is embedded; also the
rock to which one end of a crystal is attached.
Metamorphic rock—rock that has been changed from its original state by
heat, pressure, chemical action, or some combination of these
factors.
Millimeter—¹/₁₀ centimeter; approximately ¹/₂₅ inch.
Mineralogy—the science concerned with the study of minerals, including
their occurrence, composition, forms, properties, and
structure.
Monoclinic—a crystal system in which the crystal faces are described
in relation to three intersecting unequal axes, two of which
are at right angles and the third inclined.
Oligocene—a division of geologic time, estimated to be the time from
40 to 28 million years ago; part of the Cenozoic era.
Opaque—does not transmit light.
Orbicular—containing orbs or spherical or eye-like markings or
structures.
Orthorhombic—a crystal system in which crystal faces are referred to
three unequal intersecting axes at right angles.
Pavilion—the portion of a faceted gem below the girdle (fig. 6).
Pegmatite—a body of coarse-grained intrusive igneous rock, commonly
lens or dike shaped.
Perthitic—a plaid-patterned structure resulting from intermixture of
soda- and potash-rich feldspars.
Phantom crystal—a crystal outline seen within another crystal, mostly
due to entrapping of inclusions during the crystal’s growth.
Pleochroism—the property of transmitting different colors of light in
different crystallographic directions.
Point—a unit of weight equal to ¹/₁₀₀ (0.01) carat.
Porous—containing pores or void spaces.
Precambrian—a division of geologic time, estimated to be all of
geologic time prior to 550 million years ago; the time before
the Paleozoic era.
Preform—a gemstone that has been ground to a rough outline of the
finished shape of a gem.
Rhyolite—a fine-grained extrusive or shallow intrusive igneous rock of
approximately the same composition as granite.
Rough—uncut, not worked by a lapidary, not cut and polished.
Schist—a metamorphic rock that contains an abundance of oriented platy
minerals that enable the rock to be split with relative ease
parallel to the flat surfaces of the platy minerals.
Silicified—replaced by or containing a large amount of quartz or
silica.
Skill facet—a term often used for the pavilion girdle facets of the
standard American brilliant cut (fig. 6).
Specific gravity—the weight in air divided by the loss of weight in
water at a given temperature, or the weight of an object in
air divided by the weight of an equal volume of water; also
called relative density; the most commonly used standard
temperature for this measurement is 4° C. or 39.2° F.
Star facet—one of the eight facets surrounding the table facet of a
standard American brilliant cut (fig. 6).
Step cut—a mode of faceting in which the surface of the gem is covered
by a series of square or rectangular facets; stones thusly cut
are usually square, rectangular, or irregular with straight
sides in outline.
Streak—the color of a mineral when finely powdered; usually determined
by rubbing the mineral against a piece of unglazed porcelain.
Symmetry—the number, location, and balanced arrangement of crystal
faces in reference to the crystallographic axes or other
crystallographic planes or directions.
Synthetic gem—a gemstone manufactured by man that has approximately
the same chemical composition and properties as a natural
gemstone.
Table facet—the large horizontal facet found on the crown of many
gems, often called simply the table (fig. 6).
Tenacity—the resistance of minerals to breakage, described by such
terms as malleable, ductile, sectile, and brittle (p. 6).
Termination—the end of a crystal that is completely enclosed by
crystal faces, the crystal end that is not attached to the
matrix.
Tertiary—a division of geologic time, estimated to be the time from 60
to 1 million years ago; the Tertiary includes the Paleocene,
Eocene, Oligocene, Miocene, and Pliocene epochs (from oldest
to youngest).
Tetragonal—having four angles; a crystal system in which the crystal
faces are referred to three axes at right angles to each
other, two of which are equal and the third longer or shorter.
Translucent—allowing the passage of light but diffusing it
sufficiently so that objects on the other side cannot be
clearly distinguished.
Transparent—clear, allowing free passage of light so that objects on
the other side can be readily distinguished; opposite of
opaque.
Triclinic—a crystal system in which the crystal faces are referred to
three unequal axes, none of which are at right angles.
Tumbling—a process of polishing irregularly shaped gemstones (p. 17).
Vein—a tabular, irregular, or twisting mineral deposit that is thin in
relation to its length and breadth, usually the result of
solution or hydrothermal activity.
Vitreous—having luster, general appearance, or physical properties
similar to glass.
Vug—an unfilled rock cavity, commonly lined with crystals; may later
become filled by minerals owing to solution or hydrothermal
activity.
SELECTED REFERENCES
Anderson, B. W. (1948) Gem testing: Emerson, New York.
Baker, C. L. (1935) Metallic and non-metallic minerals and ores
(precious stones), _in_ The geology of Texas, Vol. II, Structural and
economic geology: Univ. Texas Bull. 3401, Jan. 1, 1934, pp. 568-569.
Barnes, V. E. (1940) North American tektites: Univ. Texas Pub. 3945,
Dec. 1, 1939, pp. 477-582.
Dake, H. C., Fleener, F. L., and Wilson, B. H. (1938) Quartz family
minerals: Whittlesey House, McGraw-Hill Book Company, Inc., New York.
Ford, W. E. (1932) A textbook of mineralogy (4th ed.): John Wiley and
Sons, Inc., New York.
Kraus, E. H., and Slawson, C. B. (1947) Gems and gem materials (5th
ed.): McGraw-Hill Book Company, Inc., New York.
Kunz, G. F. (1892) Gems and precious stones of North America (2d ed.):
Scientific Publishing Company, New York.
Pough, F. H. (1953) A field guide to rocks and minerals: Houghton
Mifflin Company, Boston.
Simpson, B. W. (1958) Gem trails of Texas: Granbury, Texas.
Sinkankas, John (1955) Gem cutting: D. Van Nostrand Company, Inc.,
Princeton, New Jersey.
—— (1959) Gemstones of North America: D. Van Nostrand Company, Inc.,
Princeton, New Jersey.
Smith, G. F. H. (1958) Gemstones (13th ed.), revised by F. C. Phillips:
Methuen and Company, Ltd., London.
Sperisen, F. J. (1950) The art of the lapidary: The Bruce Publishing
Company, Milwaukee, Wisconsin.
Sterrett, D. B. (1913) Gems and precious stones, _in_ Mineral resources
of the United States, Calendar Year 1912, Part II, Non-metals: U. S.
Geol. Survey, pp. 1023-1060.
Plate I
[Illustration: A
Gem-quality celestite crystals from Travis County, Texas. Twice natural
size. Lower portion of the crystals is colorless; the tips are dark
blue.]
[Illustration: B
Opalized wood from the Texas Gulf Coastal Plain. Specimen at left is
rich brown and tan; specimen at right is fossil palm wood and is black,
reddish brown, and white. One-third natural size.]
Plate II
[Illustration: A
Gem-quality garnet crystals and faceted gem from Gillespie County,
Texas. Natural size.]
[Illustration: B
Labradorite from Brewster County, Texas. Both stones are pale yellow.
One and a half times natural size.]
Plate III
[Illustration: A
Pink microcline crystal from Burnet County, Texas.]
[Illustration: B
Smoky quartz from Burnet County, Texas. Natural size. Colorless crystal
at center back is included for color comparison.]
Plate IV
[Illustration: Polished agate from gravels of the Rio Grande near
Zapata, Zapata County, Texas. Bands are blue and gray; other inclusions
are brown, yellow, and reddish. One and a half times natural size.]
Plate V
[Illustration: A
Texas tektites (bediasites) showing variety of surface features. Natural
size.]
[Illustration: B
Topaz crystal from a pegmatite dike near Streeter, Mason County, Texas.
Natural size. Measurements: 1½ by 1⅝ by 3 inches; weight: 194 grams (970
carats); pale blue; mostly gem quality.]
Plate VI
[Illustration: A
Topaz from stream gravels near Streeter, Mason County, Texas. Natural
size. Left to right: colorless worn pebble; emerald-cut pale-blue topaz,
weight 10 carats; pale-blue worn pebble, weight 205 carats; step out
sky-blue topaz, weight 13 carats; pale-blue worn pebble.]
[Illustration: B
Tourmaline crystals in schist from Llano County, Texas.]
Index
A
actinolite: 26
agate: 20, 28, 38
agatized wood: 27
allanite: 21
almandite: 22
amazonite: 23
amazon stone: 23
amber: 18
amethyst: 25
Amethyst Hill: 25
amorphous gemstones: 9
andalusite: 30
Arkansas: 19
Armstrong County: 21
augite: 18, 28
B
Baringer Hill, Llano County: 21, 26
baroque pearls and/or stones: 17, 25
bediasite (tektite): 28-29, 39
beryl: 18
Big Bend National Park: 27
biotite: 23
Blanco County: 18, 22
Brazos County: 29
Brazos River: 25
Brewster County: 18, 23, 24, 25, 26, 27, 28, 31, 36
brilliancy: 5
brilliant cut, standard American: 13, 15, 16
Brown County: 19
Burleson County: 29
Burnet County: 20, 22, 23, 25, 26, 30, 37
C
cabochon gems: 10-12
Caddo Lake: 25
carbuncle: 22
carnelian: 27
cassiterite: 23, 29
celestite: 19, 35
chalcedony: 27
geodes: 26
chuck: 15, 17
citrine: 25-26
cleavage: 6, 13
coal: 22
Coke County: 19
color: 5
Colorado River: 25
Concho River: 25
crown girdle facets: 16, 17
crown of gemstone: 15, 16
crystals: 7-9
crystal systems: 7
crytolite: 21
Culberson County: 22, 23, 25, 26, 31
culet: 13
cutting and polishing: 10-17
cutting lap: 13
D
DeWitt County: 29
diamond: 19
saw: 10, 11
diaphaneity: 5
dispersion: 6
dopping: 12, 13
dop-stick: 12, 15, 17
dop-wax: 12, 15, 17
dravite: 30
durability: 6
Duval County: 21, 24
E
El Paso County: 22, 30, 31
emerald cut: 15
epidote: 19-20
F
facet, kinds of: 13
main: 16
skill: 16
table: 13, 14
faceted gems and/or stones: 10, 13-17
Fayette County: 20, 29
fergusonite: 21
fire: 5
Fisher County: 19
fluorite: 20, 21
Foard County: 19
fossil wood: 20-21, 22
fracture: 6
Franklin Mountains: 18
G
gadolinite: 21-22
garnet: 20, 22, 36
gemstones, by kinds: 18-31
geodes, celestite: 19
Georgia: 29
Gillespie County: 18, 22, 23, 25, 26, 30, 36
girdle facets: 16
gneiss: 22
Gonzales County: 20, 29
grain: 25
gram: 7
graphite: 30
Grimes County: 29
grinding: 12
Guadalupe River: 25
Gulf Coast: 25
Gulf Coastal Plain: 18, 20, 22, 35
H
hardness: 6
Hudspeth County: 18, 20, 22, 24, 25, 26, 27, 28, 31
I
index of refraction: 5
J
jasper: 27-28
Jeff Davis County: 26, 27, 28, 31
jet: 22
K
Kinney County: 28
L
labradorite: 23, 36
Lake Buchanan: 21
Lampasas County: 19
lap plate: 13
Lavaca County: 29
Lee County: 20, 29
lignite: 22
Live Oak County: 21
Llano County: 18, 19, 20, 21, 22, 23, 25, 26, 30, 31, 40
Llano River: 25
luster: 5
M
Madeira topaz: 29
Mason County: 20, 22, 23, 26, 29, 30, 39, 40
Massachusetts: 29
Maverick County: 18
microcline: 20, 21, 23-24, 29, 37
Mohs scale of hardness: 6
moonstone: 28
Mount Bonnell: 19
muscovite: 23
N
natural glass: 18, 24, 28
Needle Peak, Presidio County: 27
nivenite: 21
Nolan County: 19
Nueces River: 25
O
obsidian: 24, 29
Oklahoma: 29
opal: 20, 24
opalized wood: 35
orbicular jasper: 28
ounce: 7
P
Packsaddle Mountain: 23
Packsaddle schist: 30
palm wood: 21, 35
Palo Duro Canyon: 21
pavilion: 13, 16
facets: 16
girdle facets: 16, 17
pearl: 24-25
pegmatites and/or pegmatite dikes: 18, 20, 21, 22, 23, 26, 29, 39
petrified wood: 26
phantom crystals: 26
pistacite: 20
pleochroism: 5
point: 7
polishing: 17
lap: 13, 16
preformed stone: 16
preforming: 15
Presidio County: 20, 22, 24, 25, 26, 27, 28
properties of gemstones: 5-7
Q
quartz: 20, 21, 23, 25-28, 29, 30
smoky: 38
R
radioactive elements: 22
radioactivity of gadolinite: 21
rarity: 6
Reeves County: 26, 27
Rio Grande: 25
gravels of: 21, 27, 38
Valley: 27
rock crystal: 26
rose quartz: 26
S
Sabine River: 25
sanding: 12
sanidine: 28
sawing: 10
scheelite: 20
schorl: 30
size: 7
“skill” facets: 16
“slab” of gem materials: 11
Smithsonian Institution: 21
smoky quartz: 23, 26, 37
smoky topaz: 29
Spanish topaz: 29
specific gravity: 7
spinel: 18, 28
star facets: 17
step cut: 15
streak: 6
synthetic gems: 7
T
table facet: 13, 15
tektite (bediasite): 28-29, 39
tenacity: 6
thorogummite: 21
topaz: 23, 26, 29-30, 39, 40
quartz: 29
tourmaline: 26, 29, 30-31, 40
Town Mountain, Llano County: 26, 30
transparency: 6
Travis County: 19, 35
Trinity River: 25
tumbled gems: 17
turquoise: 31
U
U. S. National Museum: 30
V
value of gemstones: 6, 7
Val Verde County: 24
valverdites: 24
Van Horn, Hudspeth County: 27
W
Walker County: 29
Washington County: 20
Webb County: 21
weight, units of: 7, 25
Z
Zapata County: 38
Transcriber’s Notes
--Silently corrected a few typos.
--Renumbered figures 6 and 7 (and references to them) to correspond to
their order in the printed book.
--Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
--In the text versions only, text in italics is delimited by
_underscores_.
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