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.