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 GRAVITY | INDEX OF REFRACTION | COMMON 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, Fe)₃Si₃O₁₃ | 6.0-7.0 | 3.25-3.50 | 1.72-1.77 | yellowish green, brownish green |
| Fluorite | CaF₂ | 4.0 | 3.0-3.25 | 1.434 | colorless, violet, yellow, green |
| Garnet (Almandite) | Fe₃Al₂(SiO₄)₃ | about 7.5 | 4.25 | about 1.83 | red, deep red, brownish red |
| Labradorite | NaAlSi₃O₈ 50% to 30% CaAlSi₃O₈ 50% to 70% | 6.0-6.5 | about 2.6 | about 1.56 | yellowish, grayish |
| 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 (Crystalline) | SiO₂ | 7.0 | 2.65-2.66 | 1.544-1.553 | colorless, violet, yellow, brown |
| Tektite (Bediasite) | natural glass | 5-6 | 2.33-2.44 | 1.48-1.52 | dark brown, 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.
Fig. 1. Typical crystal form of three common Texas gemstones.
GARNET TOURMALINE QUARTZ