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