Paul E. Desautels

“ The effect of a gem on light may be more than the production of color. Several of the so-called phenomenal stones are prized for other effects. Holes, bubbles, and foreign particles, when properly aligned in parallel groupings, can produce interesting light effects. The play of colors of opal and labradorite, the chatoyancy or silky sheen of tiger’s-eye and cat’s-eye, the opalescence or pearly reflections of opal and moonstone, and the asterism or star effect of rubies and sapphires are caused by the reaction of light to minute inclusions or imperfections in the gemstone. ”

“ The action of a gemstone upon the light which strikes its surface and is either reflected or passed through it sometimes results in highly desirable effects that enhance its beauty and aid in its identification. ”

“ To be used as a gem, a mineral species must have durability as well as beauty. Lack of durability eliminates most minerals as gems, although some relatively fragile gem materials such as opal are prized because of their exceptional beauty. Actually, some gem materials are not minerals at all. Pearl, amber, jet, and coral are formed by living organisms.
In the National Gem Collection, the Smithsonian Institution has assembled a large representation of all known gem materials. ”

“ A number of mineral species have produced cut gemstones that fulfill every necessary requirement of beauty, durability, and rarity, but their popularity and commercial success have been sharply limited because of insufficient supply. In some cases of even adequate supply such gemstones do not compete with other, more plentiful kinds that exhibit the same characteristics. The scarcity of these minerals does not diminish their standing as potential gem material—it merely points up the effect of accidental natural distribution of these species. ”

“ Spinel is an oxide of magnesium and aluminum, and it is not related to ruby. However, because its hardness (8) is only slightly less than that of ruby and its brilliance is about equal to that of ruby, spinel makes an excellent substitute for that gem. Also, because it is more plentiful, spinel costs much less. It is interesting that red spinel, like ruby, gets its color from the presence of traces of chromium. ”

“ The study of all these species of gem minerals constitutes modern gemology—a specialized branch of the science of mineralogy.
With the few exceptions already noted, all gems are minerals found in the earth’s crust. A mineral is a natural substance having a definite chemical composition and definite physical characteristics by which it can be recognized. ”

“ Separation occurs because the various colors, or wavelengths composing white light passing through the gem, are each bent or refracted a different amount. Red is bent least, followed in order by orange, yellow, green, blue, and violet, which is bent most. This characteristic of being able to produce flashes of color, as seen prominently in diamond, is known as dispersion or fire. Quartz and glass have low dispersion, and hence they make poor diamond substitutes. Some of the newer synthetic gemstones, such as titania, have extremely high dispersion, with resulting fire. ”

“ Gemstones such as beryl and sapphire that depend on impurities for their color are said to be allochromatic; others, such as peridot and garnet, which are highly colored even when pure, are said to be idiochromatic. The color of a gem is further described according to its hue, tint, and intensity. Hue refers to the kind of color, such as red, yellow, green, etc.; tint refers to the lightness or darkness of the hue; and intensity refers to vividness or dullness. Throughout history, the most popular colored stones have been those with hues of red, green, or blue of dark tint and high intensity. ”

“ We are aware of a difference in weight when we compare lead and wood, yet it would not always be correct to say that lead weighs more than wood, for a large piece of wood can weigh more than a small piece of lead. Only by comparing equal volumes of these materials can the extent of the weight difference be clear and unmistakable. Diamond is 31⁄2 times heavier than the same volume of water, so its specific gravity is 3.5. Since each species of gem has its own specific gravity, which can be determined without harming the stone, this standard of comparison is a valuable aid in identifying gems. ”

“ For over 200 years mineralogists have been devising techniques for producing synthetic minerals in the laboratory, and attempts have been made, sometimes with considerable success, to apply these techniques to the production of synthetic gemstones. To qualify as a synthetic gemstone the man-made product must be identical chemically and structurally with its natural counterpart. Sapphire, ruby, spinel, emerald, and rutile in gem quality have been brought to commercial production. ”

“ The world’s largest diamond deposits are in Africa, and names such as Congo, Sierra Leone, and the Union of South Africa bring to mind colorful legends of fabulous discoveries of diamond. Smaller deposits are found in South America—in Brazil, British Guiana, and Venezuela—and in Asia. Even in the United States some diamonds have been found. ”

“ In the distant past, moonstone, topaz, and other white stones were believed to be in sympathy with the Moon, diamond and ruby with the Sun, jasper and emerald with Mars, amethyst, topaz, and emerald with Venus, carnelian, topaz, and amethyst with Jupiter, turquoise and sapphire with Saturn, and rock crystal, agate, and emerald with Mercury. Since Uranus, Neptune, and Pluto were unknown to the ancients, these planets have not been represented by gemstones.
Of special interest to the American public are birthstones. ”

“ Crystals of quartz varieties that are opaque or that contain visible inclusions normally are cut as cabochons to take advantage of the body color or to make the inclusions more visible. Crystals of the transparent varieties are fashioned in any of several cutting styles, depending on whether it is desired to take maximum advantage of color or of brilliance. Because of its availability in fairly large, flawless pieces in various colors, quartz has been used extensively in carving. The Chinese have excelled in carving large, ornate objects of rock crystal. ”

“ Both ruby and sapphire, which are second only to diamond in hardness, are of the mineral species corundum, an oxide of aluminum. They are identical in all characteristics except color. Most corundum is opaque, and it is mined in large quantities for use as an abrasive. In a few places, such as Moguk in Upper Burma and in Ceylon, clear corundum is found that is suitable for use as a gem. ”

“ The ancient Chinese believed that pearls originated in the brain of a dragon. We now know, of course, that pearl is created by a secretion of a mollusk. Very few mollusks have the ability to produce the fine mother-of-pearl used in the jewelry trade, and even among those that can, very few produce pearls with iridescence, or orient, as it is known in the trade. Only two genera, the pearl oyster (Margaritifera) and the pearl mussel (Unio) are important sources of the gem. Edible 32 oysters rarely produce pearls, and when they do, the pearls are of poor quality. ”

“ There are records of glass and ceramic imitations of gems as early as 3000 B.C. Certainly, the world gem markets today are flooded with man-made gems. There even has been developed a laboratory process for growing a coating of synthetic emerald on the surface of a faceted stone of natural colorless beryl. The recut gem looks like a natural emerald, and it has natural inclusions that totally synthetic emeralds lack. ”

“ Certain of the light beams passing into a brilliant-cut diamond produce colorless brilliance by being reflected back out of the stone 13 through the table by which they entered. Other light beams, emerging through inclined facets, are split up by dispersion into the rainbow, or fire, effect so prized in diamonds. A stone that has been cut too wide for its depth, with incorrect facet angles, will look large for its weight but its brilliance and fire will have been drastically reduced. ”

“ Also, the seams in rock sometimes are cut so that the thin layer is exposed on a 45 thicker backing of the adjoining rock. Precious opal, or gem opal, is classified as white opal when the color flashes are in a whitish or light background, black opal when the background material is gray, blue-gray, or black, and fire opal when the background is more translucent and red, reddish orange, or reddish yellow. ”

“ Jadeite is highly prized, and when it occurs as emerald green it is considered one of the most valuable gemstones. This kind of jade is found in many places, but the most important occurrence is in Upper Burma. Nephrite, a more common species, is a calcium magnesium iron silicate belonging to a group of rock-forming minerals known as amphiboles. The color varies from white to a dark spinach green and black. Among the places where nephrite occurs are New Zealand, Turkestan, Siberia, Alaska, China, Silesia, and certain parts of the western United States, notably in Wyoming and California. ”

“ Their value depends on their color, clarity, cut, and carat weight. Most costly are those called fancies, which have a distinct color such as blue, pink, green, or deep yellow.
31 PEARL
Pearl is included among gemstones only because it is a beautiful object used as jewelry. As has been noted, pearl is not mineral because it is formed by the action of a living organism. However, the pearl has long occupied an important position among jewels, and it is unique in requiring no lapidary art to enhance its beauty. Nature has perfected pearls. ”

“ Since each species of gemstone differs in its characteristics, each must be treated somewhat differently as to sawing and lapping speeds, kind of lap, and choice of abrasives. Because of the greatly increased interest in gem cutting as a hobby and the large number of amateur cutters, a substantial market has developed in the United States for lapidary supplies and equipment. New kinds of machinery, new abrasives, and new kinds of saws and laps are introduced regularly. ”

“ The Portuguese Diamond, weighing 127 carats, is the 13th largest cut diamond on record. More unusual, it is from Brazil, and is thought to have been part of the Portuguese crown jewels. In addition to its brilliant color flashes, it has a slight milky fluorescence that causes it to “glow” even in artificial light. (Actual size.)
Diamonds vary from colorless to black and from transparent to opaque. As they come from the mines, they are graded into two groups, gem and industrial. ”

“ The color of tiger’s-eye arises from the partial alteration of the asbestos to yellow-brown iron oxides before it is replaced by quartz. Inclusions of rutile, tourmaline, or actinolite needles may produce attractive patterns in quartz, but they do not always cause chatoyancy. The material containing such inclusions 51 is called sagenitic quartz, or it may be descriptively named, such as rutilated quartz, tourmalinated quartz, and so forth. Sagenitic quartz is usually cut as cabochons rather than as faceted stones since the inclusions are of greater interest than the quartz itself. ”

“ Because of its hardness (about 8) , vitreous luster, beautiful color, and rarity, emerald always has been highly prized as a 40 gem. Fine-quality emeralds may be more costly than fine diamonds. Other kinds of beryl have the same physical properties as emerald, but since they are less rare their relative value is lower.
Synthetic emerald of high gem quality has been marketed successfully. A synthetic substitute for aquamarine is also available; it is really a synthetic blue spinel. ”

“ Ceylon, Burma, Madagascar, and Brazil also have furnished some gem spessartine.
The chromium garnet, uvarovite, generally is too poor in quality for cutting. Uvarovite crystals, which are emerald green in color, occur in only small sizes. They are found mostly in Russia, Finland, and California.
Grossular varies in color. It occurs chiefly in some shade of red, green, yellow, or brown, depending on the impurities present. When pure, grossular is colorless. A kind of grossular called hessonite has an attractive cinnamon color, and it is found mainly in Ceylon. ”

“ Like ruby and several other gemstones, spinel is found chiefly in the gem gravels of Ceylon, Burma, and Thailand. Appreciable amounts of spinel occur in the Ceylon gem gravels as worn, rounded pebbles of many colors. In the Burmese gravel deposits the spinel is often found as well-formed octahedral crystals. Near Moguk, in Burma, spinel has been found in its original position in the limestone rocks as well as in the eroded stream deposits. ”

“ The kunzite and hiddenite varieties of spodumene show strong pleochroism, or the ability to show three different colors when viewed in the direction of different axes. Some of the large Brazilian kunzite crystals mined in the early 1960’s have 62 an intense rose-violet color when viewed along the long axis of the crystal but have pale blue-violet and pale tan colors when viewed from the other two directions. When heat treated, or exposed to strong light, this Brazilian kunzite loses its tan and bluish colors but retains the intense rose-violet. ”

“ Garnet has a hardness (about 7) suitable for gemstone material and a fairly high refractive index (1.74 and above) .
Ruby red pyrope is the most popular variety of garnet. It is found in Bohemia, in Czechoslovakia, where it occurs as small, poorly shaped crystals. Red pyrope also is found in Africa, where it is called Cape ruby, and in Arizona, where it is sold as Arizona ruby. Another kind of pyrope called rhodolite is noted for its soft, rosy purple color. Actually, rhodolite is one of the intermixed garnets with a composition somewhere between pyrope and almandine. ”

“ The opal, by contrast, is relatively fragile, and it 4 depends mainly on its rarity and its beautiful play of colors to be considered gem material.
When a gem material, as found in nature, has at least a minimum number of the necessary qualities, it is then the task of the lapidary, or gem cutter, to cut it and polish it in such a way as to take greatest advantage of all its possibilities for beauty and adornment. ”