Part 2

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.

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.

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.

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.

CABOCHON DOP-WAX DOP-STICK