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A gem is a precious or semiprecious stone or other substance that may be used to make jewelry. The term "gem" is also used to refer to such stones after they have been cut and polished for use in jewelry. Most gems, such as diamonds, rubies, and emeralds, are minerals, that is, naturally occurring, homogeneous inorganic substances. A few, however, such as pearls, amber, and coral, are organic in origin; that is, they are derived from formerly living organisms.
For many thousands of years, gems have been prized for their beauty and value. In ancient times they were used not only for personal adornment but also as charms, to which primitive tribes ascribed magical or curative powers. Like other things for which there is a steady and universal demand, gems are valued as a means of concentrating wealth. At the present time the most highly valued gem in most of the Western world is the diamond. At other times and in different places, however, other gems have been preferred. For example, lapis lazuli was highly valued by the Egyptians. The ruby has long been favored in India, and jade has been traditionally preferred in China.
Characteristics of Natural Gems
The value of a gem is determined largely by its beauty. The beauty of a gem depends upon several factors, including luster, brilliancy, color, and fire. In addition, a gem's value depends upon its hardness and its rarity.
Luster. The character of the light reflected from the surface of minerals and other substances is called luster. It is one of the means of distinguishing substances. The luster of diamonds is called adamantine. Quartz and other gems that look glassy are said to have a vitreous luster. Pearl has a pearly luster, amber a resinous luster, and turquoise a waxy luster. Other terms used to describe luster include metallic, silky, or greasy.
Brilliancy. Brilliancy depends upon the way in which a gem stone is cut. When light enters a gemstone, it is refracted, or deflected, internally. The amount of this refracted light that reemerges from the stone determines the stone's brilliancy. The higher the percentage of refracted light that emerges from the gem-stone, the more brilliant will be the stone.
Color. Color in most gems is caused by the presence of small amounts of impurities, usually metallic oxides. The impurities selectively absorb certain wavelengths of the light that strikes them. Light reflected from the gem lacks these wavelengths and is therefore colored. For example, rubies contain chromium oxide, which absorbs the green wavelengths of white light. Thus the light reflected by a ruby appears red.
Some gems, particularly opals, display shifting colors similar to those seen in an oil slick or on the surface of a soap bubble. The colors, which are caused by minute air films or faults within the opal, seem to change each time the position of the stone is shifted slightly.
Fire. Fire is a play of flashes of color produced by colorless and transparent gems, such as the diamond. It is caused by dispersion, the same phenomenon that occurs when white light is split into a spectrum by a glass prism. Dispersion occurs because each component of the white light that enters the gem is refracted, or bent, by a different amount, and the light spreads out into its component colors. The greater the dispersion, the more the components of white light are separated and the easier it is to see the colors.
The degree to which light is refracted when it enters a gem is measured by the refractive index of the gem. The refractive index of the diamond is greater than that of most other gems. Its high refractive index and high dispersion give a properly cut diamond a brilliant display of fire, for which it is highly valued. Zircons and garnets also possess a high degree of fire.
Hardness. The harder a stone is, the longer it will withstand wear from usage. All the most precious stones, such as diamonds, rubies, sapphires, and emeralds, are extremely hard. Their relative hardness is usually measured by means of the Mohs scale. This is an arbitrary scale in which the diamond, the hardest-known mineral, is assigned the number 10, and talc, the softest mineral, is assigned the number 1. A gem can scratch any other gem that occurs below it in the Mohs scale, but it can be scratched only by those that occur above it.
Rarity. No matter how beautiful and durable a gem may be, its value will be affected by its abundance. Certain semiprecious stones, such as the garnet, are as beautiful and durable as many precious gems, but they are less valuable because they are not as rare.
Gems in their natural state often bear little resemblance to the stones used in jewelry. Many are so dull and colorless that only an expert can recognize them. Their beauty is brought out only when they are cut.
There are many ways in which a stone may be cut, and the method that is used depends upon the kind of stone and the purpose for which it is intended. In every case the purpose of cutting is to bring out the hidden brilliancy, color, and fire of the stone. Gem-stones are cut by two principal methods, which are known as the cabochon cut and the facet cut.
The cabochon cut is usually used for opaque or semitransparent stones, such as the opal and the turquoise. Gems cut by this method have rounded, smooth surfaces.
The facet cut has smooth, flat facets, or faces, angled and polished so that the maximum amount of light will be refracted from the gem. Large stones may be cut with more than 100 facets. The facet cut is almost always the method used for brilliant stones, such as the diamond.
Formation and Structure
Gem minerals have been formed in many different ways over a period of many millions of years. Some rubies, sapphires, and emeralds had their origin in metamorphic rocks. These rocks were formed deep beneath the earth's surface, and the high temperatures and pressures that existed there caused the gem minerals to crystallize. Diamonds also crystallized deep within the earth's crust at high temperatures and pressures, but from a molten rock material. Some gem minerals, such as topaz, tourmaline, and aquamarine, were formed when pegmatite, a coarse-grained granite, crystallized. Other gem minerals, such as opal, were formed by crystallization from solutions containing dissolved mineral matter.
No matter how they were formed, nearly all gems are crystals. Gem crystals may vary in size from microscopic flakes weighing little to giant crystals weighing several tons.
For many centuries, attempts have been made to manufacture stones that would be indistinguishable from diamonds and other precious gems. Only in the last 100 years, however, has any success been achieved. There are three kinds of manufactured gems: treated gems, synthetic gems, and imitation gems.
Treated Gems. Treated gems are natural stones that have had their appearance changed by dyes, heat treatment, or radiation. The colors of some stones can be brightened or changed entirely by soaking them in certain solutions. For example, if agate is soaked in honey and then in concentrated sulfuric acid, it turns black. Soaking it in other solutions gives it a variety of pleasing colors in complex patterns.
Other gems change color when they are heated. For example, topaz becomes pink when heated to about 720° F. (400° C.). Amethyst becomes light or dark yellow when heated, and red or brown zircons often become an attractive dark blue. The color of certain gems may also be changed by subjecting them to nuclear radiation. For example, colorless diamonds turn green when exposed to radiation, and colorless quartz becomes smoky brown.
Synthetic Gems. Synthetic gems are artificially created products that have the same chemical composition and physical properties as natural gems. In every case, however, the synthetic gems differ from natural ones in minor characteristics. Since 1900, scientists have succeeded in synthesizing nearly all the precious stones in the laboratory. Synthetic rubies and sapphires were first produced in 1902 by a commercial process invented by the French scientist Auguste Verneuil. Emeralds were first synthesized in 1930 by scientists at the I. G. Farben Company in Germany. After many years of fruitless attempts by different scientists, synthetic diamonds were finally made by the General Electric Company in 1955. The process involved pressures of nearly 2 million pounds per square inch (140,775 kg per sq cm). However, the diamonds produced in this way are very small and are suitable only for industrial use. In 1970, gem quality diamonds weighing over one carat each were synthesized by scientists at General Electric. High temperatures and pressures were required, together with a metal such as iron or nickel to speed up the reaction.
Imitation Gems. Imitation gems look like natural gems but differ from them in composition. Most imitation gems are made from glass. Paste, a special type of glass with a high lead content, is often used. Since World War II, plastic has been widely used to imitate such gems as pearls and ambers.