DIAMOND

Diamond is the king of gems. It is a form of pure carbon, and it is the hardest substance known; only diamond will cut diamond. It is interesting that the humble graphite, its close relative, is also pure carbon, but graphite is so soft that it is used as a lubricant and for making the “lead” in pencils.

The ancients believed diamond to be indestructible, and even today many people believe that diamond cannot be broken. Despite its great hardness, however, diamond is not exceptionally tough, and it can be split along what diamond cutters call itsgrain.

The diamond’s high brilliance results from its very high refraction, or ability to bend light, and its fire is caused by its high dispersion, or ability to divide light into its rainbow colors. However, only in properly cut stones are diamond’s brilliance and fire developed to their maximum.

At great depths in the crust of the earth and under conditions of very high pressure and temperature, diamonds form in pipe-like bodies of kimberlite, a heavy dark rock consisting primarily of two minerals, pyroxene and olivine. In South Africa diamonds are mined from the kimberlite, but they also are recovered there and elsewhere from beds of sand and gravel where they have accumulated after being released from their mother rock by erosion.

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.

India was the most important source of diamond until 1728, when discoveries were made in Brazil. Among the important large diamonds found in India were the Koh-i-noor, the GreatMogul, and, very likely, the Hope Diamond. Like India, Brazil in turn declined as a major source of diamond with the discovery and efficient recovery of large quantities in South Africa.

The Hope Diamond, because of its long and dramatic history and its rare deep-blue color, is probably the best known diamond in the world. By speculation, the Hope is linked to the famous “French Blue,” which was brought to France from India in 1668 to become part of the crown jewels of Louis XIV. The French Blue was stolen in 1792 and never recovered, but in 1830 an extraordinary 44.5-carat blue diamond—presumably cut from the missing gem—came on the market. It was purchased by Henry Thomas Hope of England and became known by its present name. In 1949 the gem was acquired from the estate of Mrs. Evalyn Walsh McLean by Harry Winston Inc., of New York. Ten years later, Harry Winston, Inc., presented the gem (shown here in actual size) to the Smithsonian Institution.

Diamonds are extremely rare even in diamond mines. For example, the famous South African mines contain only one part of diamond in more than 14 million parts of worthless rock. In spite of this, more than three tons of gem- and industrial-quality diamond were mined in 1963.

Among the British crown jewels is a cut diamond weighing 530.20 carats (more than 3¾ ounces), one of several stones that were cut from the largest gem diamond ever discovered.The rough stone, known as the Cullinan Diamond, weighed 3106 carats (almost 1¾ pounds) when it was found at the Premier Mine in South Africa in 1905.

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. Those whose color, imperfection, or shape make them useless as gems—more than 8 out of every 10 carats mined—are used in industry. Diamonds of industrial quality also are produced synthetically, and these are used primarily in the manufacture of grinding wheels.

The best gem diamonds are flawless and are colorless or slightly blue. 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.

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.

The strand of matched pearls was presented to President Van Buren by the Imam of Muscat. The three baroque (irregularly shaped) pearls are freshwater pearls from the Wabash River in Indiana.

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, ororient, 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. Edibleoysters rarely produce pearls, and when they do, the pearls are of poor quality.

The shells of pearl-producing mollusks are composed of layers of calcium carbonate in the form of either calcite or aragonite. These layers, cemented together with an organic substance known as conchiolin, are known as nacre. The layer closest to the animal is deposited in tiny overlapping patches, producing an iridescent effect caused by the interference of light rays reflected from the plates making up the nacre. The same material coats the surface of a gem pearl.

Seldom does a mollusk live out its time without attack by creatures boring through its shell, or without intrusion through the normal shell opening of tiny parasitic worms, sand, or other irritants. Usually inert particles are forced against the inside of the shell, where they are covered with layers of pearl that fasten them to the shell. This is the source of mostblister pearls. When the irritant remains in its fleshy part, the mollusk deposits a protective shell of pearl to cover it completely, and a spherical pearl may result. Pearls of less-symmetrical shape, calledbaroques, are more common.

The value of a pearl depends on its shape, color, orient, and size. Pearls of highest value are white with a faint tinge of pink or yellow, possess fine orient, are round, and are free of surface blemishes. The grading of pearls for color requires considerable experience to detect delicate differences. Various classification names, such as “rosée” for delicate pink shades, are used. Fancy colored pearls are those with a strong yellow, bronze, pink, green, blue, or black color. Grading for shapes, which differ markedly, is easier. Spherical pearls are usually drilled for beads; pear-shaped or drop pearls are used in earrings and pendants; and “boutons” or button-shaped pearls, with one flat side, are used for ear ornaments, cuff links, and rings. Irregular, baroque pearls and tiny seed pearls are used in jewelry designs with noble metals and perhaps other gemstones.

The world’s finest pearls, calledoriental pearls, come from the fisheries of the Persian Gulf. Fine pearls also are found off the coasts of Burma, Tahiti, New Guinea, Borneo, Venezuela and western South America, and in the Gulf of California. Fresh-waterpearls of high quality, formed in pearl mussels, are found in various rivers in Europe and the United States, especially in rivers in the Mississippi Valley.

A method of growingcultured pearlshas been well developed. A mother-of-pearl bead is inserted in the oyster as an irritant, and the animal is replaced in the sea in a cage. When oysters so treated are recovered after a period of three to seven years, the beads in the harvested crop usually are found to be coated with a layer of nacre up to almost a sixteenth of an inch thick.

The cultured pearl can be identified only by the observance—through a drill-hole or by X-ray—of the mother-of-pearl core, which had been inserted in the oyster. An instrument called an endoscope, devised for rapid testing of drilled pearls, relies on a beam of strong light carried by a hollow needle. The needle is inserted into the drill hole, and as it passes through the center portion of a natural pearl a flash of light, reflected through a mirror system in the needle, is observed.

Bothrubyandsapphire, 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.

Red corundum is known as ruby. Its color, caused by traces of chromium, ranges from rose through carmine to a dark purplish red referred to as pigeon’s blood red. Rubies of this very desirable latter color often are called Burma rubies, and they are the most costly of all the corundum gems.

All gem corundum having a color other than red is sapphire. The name sapphire means blue, and this is the color most frequently associated with this gemstone. The finest sapphires are a velvety cornflower blue, and they come from Kashmir. Blue,white, yellow, gold, pink, and all the other colors of corundum are caused by the presence of slight traces of iron, chromium, titanium, and other metals present as dissolved impurities in the aluminum oxide. Frequently sapphires are found that show patches of blue and yellow, or that have alternating zones of red and blue. Pure corundum is colorless.

A piece of uncut ruby, from Burma, and five small rubies of about half a carat each, from Ceylon. All have the classic “pigeon’s blood” color. (Actual size.)

Most gem corundum comes from the Orient, at localities such as Moguk in Upper Burma, near Bangkok in Thailand, Kashmir in India, and Ceylon. Because of this primarily Asian origin, the wordorientaloften is used with the names of other gems to denote a sapphire of a particular color. For example, green sapphire sometimes is called oriental emerald, and the yellow sapphire sometimes is called oriental topaz.

The sapphires in this group vary in color from deep blue to gold, and they come from widely separated localities. The scatter of small multicolored stones came from Montana, and the magnificent 93-carat golden sapphire, encircled by the gold bracelet, came from Burma. (Slightly less than half actual size.)

There are some notable exceptions to the generally oriental occurrence of corundum. Some good-quality ruby has been found in North Carolina, and sapphire of many colors has come from Montana.

During the formation of a corundum crystal, extremely small needle-like inclusions of rutile sometimes occur in the hexagonal pattern of the host crystal. When such inclusions are arranged in this way by nature, they cause, in properly cut stones, internal reflections that produce the optical phenomenon known as asterism. The effect is that of a 6-rayed star, and the gems in which asterism occurs are known as star sapphires and star rubies. Asterism is rarer in ruby.

The Star of Asia, weighing 330 carats, is one of the finest star sapphires in the world. It is of a clear, deep blue color and has a strong, sharply defined, 6-rayed star. (Actual size.)

Cutting a star stone requires careful attention to the directions in which the cuts are to be made. Failure to align the stone properly with the axis of the crystal will produce a stone with an off-center, crooked, or dim star, or may even eliminate the star completely.

Since corundum is easily manufactured, synthetic ruby and sapphire are used extensively in jewelry. The synthetic stones can be distinguished from natural stones by microscopic examination of the kinds of inclusions and internal defects.

Beryl is probably the most widely used colored gemstone, and under its several names in the gem world it is probably the best known. When it is a rich green it is known asemerald, and when it is the blue-green of sea water it is calledaquamarine. Varieties such as the rose-pinkmorganite, golden-yellowheliodor, and colorlessgosheniteare less well known than emerald and aquamarine but are equally attractive and satisfactory gemstones.

Beryl is beryllium aluminum silicate. It frequently occurs in well-formed hexagonal crystals, and its many colors result from the presence of very small percentages of several different elements. Emerald owes its rich green color to traces of chromium, and the detection of this element is one of the means of identifying true emerald. Aquamarine, comprising the green and blue-green beryls, gets its color mainly from traces of iron. Practically all of the deep blue aquamarine available in jewelry stores results from the heat treating of greenish beryl or certain yellow-brown beryls. The stones are heated carefully to about 800° F., and the color change is permanent. The element lithium accounts for the color of pink beryl. As with aquamarine, the color of yellow beryl is now considered to be the result of traces of iron rather than uranium, as previously thought. Pure beryl is colorless.

Beryl usually is found in pegmatites, which are very coarse-grained granite rocks formed by the cooling of molten material far beneath the earth’s surface. As the rock cools and beryl and other crystals are formed, the stresses introduced are so great that the crystals frequently shatter so badly they are useless as gem material. Frequently, too, impurities are introduced during crystal formation, and consequently the gem materials are found only where the crystals were able to form without interference—such as in openings or cavities in the rock.

Tremendous beryl crystals weighing as much as several tons, but not of gem quality, have been discovered in a few localities.Large crystals of gem quality also occur in nature, and large cut stones of aquamarine and other colors of beryl are relatively common. Among the fine examples of beryl in the National Gem Collection is a remarkably large (2054-carat), flawless cut stone of rich yellow-green. This gem and others in the collection weighing 1363 carats, 1000 carats, 914 carats, and 578 carats accentuate the occurrence of large gem crystals of beryl in Brazil.

Four large cut stones, all from Brazil, illustrate the color range of beryl. Top, a 578-carat green beryl; left, a 235-carat morganite, gift of Mr. and Mrs. Frank Ix, Jr.; bottom, a 133-carat gold beryl; and, right, a 187-carat aquamarine. (Half actual size.)

The finest emeralds are not found in pegmatites. At Muzo in Colombia, the most prolific source of the finest emeralds, they occur in veins with calcite, quartz, dolomite, and pyrite. The veins cut through dark-colored, carbonaceous limestone and shale. Mining at Muzo began 350 years ago and still continues sporadically to meet market requirements. Russian emeralds occur as good-sized crystals in mica schist, a metamorphic rock. They occur there with chrysoberyl, phenakite, and common beryl. Some of the smaller stones have good color and have been cut into valuable gems. Brazil, whichproduces many extraordinary aquamarines and other beryls, has not produced quality emeralds. Periodically, over the centuries, there have been reports of new discoveries of emerald, but so far none of these has begun to rival the Muzo source in either quantity or quality of the gems produced.

This tremendous golden beryl from Brazil, weighing 2054 carats, is the largest cut beryl known of this color. Cut stones of this size that contain no visible flaws or inclusions are most unusual. (Three-fifths actual size.)

Although Brazil supplies the finest aquamarine and Colombia the finest emerald, several localities in the United States are sources of good-quality beryl of these colors. Foremost among these localities are Maine, California, and Connecticut for aquamarine and North Carolina for emerald. Morganite of pale pink to deep peach color, from California, is also notable. Various New England mines in Maine, New Hampshire, and Connecticut and the gem mines of the Pala and Mesa Grande districts of California have produced other colors of gem beryl. However, most of the beryl mined in the United States is used as an ore for beryllium, as little of it is of gem quality.

Because of its hardness (about 8), vitreous luster, beautiful color, and rarity, emerald always has been highly prized as agem. 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.

Three different cutting styles and colors of topaz. From top, a 235-carat colorless stone from Colorado, a 171-carat dark champagne-colored stone from Madagascar, and a 129-carat sherry-colored stone from Brazil. (Slightly less than actual size.)

Because yellow is the most popular color of topaz it has become customary to believe that all topaz is yellow. Also, there is a tendency to believe that all yellow gemstones are topaz. Neither belief is correct. Stones of yellow, sherry, blue, pink, and colorless topaz all make beautiful gems, and their characteristics are identical except for color. On the other hand, citrine (a yellow quartz), although entirely unrelated to topaz, often is disguised in the trade under the names Brazilian topaz, topaz quartz, or just topaz. Great numbers of stones described and sold as yellow topaz really are the much commoner citrine, which has few of the characteristics of fine topaz.

A cushion-cut topaz from Brazil that weighs 1469 carats. It is an odd shade of yellow-green.

A 3273-carat topaz of soft blue that came from Brazil. The Smithsonian Institution had this unique gem cut by Capt. John Sinkankas of California. For several years it was the largest topaz in the collection. (Both gems are shown in actual size.)

Topaz, an aluminum fluosilicate, has a hardness of 8, a vitreous luster, and a relatively high refractive index. It is found in near-perfect crystals that range in size from very small to very large, with some giants weighing as much as several hundred pounds. Most of these crystals, especially the largest ones, are colorless, a characteristic that indicates relatively high purity of composition. Although topaz gems have little fire, they take a high polish and can be very brilliant. Great care must be taken in cutting and polishing topaz because of its ready cleavage. The desired cut and high polish can be secured by avoiding excessive heat or pressure during the operation and by planning facets so that none lies exactly parallel to the cleavage direction.

Although crystals of gem-quality topaz are found in many localities, perhaps the splendid blue ones from Russia and the yellow, wine, blue, and colorless ones from Brazil are best known. Some fine topaz has been found in the United States in such widely separated areas as New Hampshire, Texas, Colorado, and California. The light, golden brown topaz from Colorado has an unfortunate tendency to fade in strong sunlight. It remains to be seen whether similar topaz coming recently from comparable occurrences in Mexico also will fade. By a system of heating and cooling, certain of the red-brown topaz crystals from Ouro Preto, Brazil, can be converted to colors ranging from salmon pink to purple red. Quick heating to high temperatures can completely remove color, and sudden or uneven cooling may cloud or crack the stone.

Opal has been admired for its great beauty since ancient times, but this gemstone lacked commercial appeal until the discovery of the Australian black opal late in the 19th century.

Opal is somewhat brittle, is sensitive to heat, and, in some cases, tends to deteriorate despite the best of care. Therefore, this stone lacks many of the physical characteristics required for an ideal gem. These deficiencies would eliminate other species from the list of gemstones, but the great beauty of its flashing and shifting color patterns has made opal increasingly popular. Even its name, coming from the ancient Sanskrit “upala,” means precious stone.

With a hardness between 5½ and 6½, opal is the softest of the more popular gems. It is sufficiently hard, however, to be used in jewelry, where its setting usually helps to protect it from shock and abrasion.

Black opal, so called because the color flashes appear against a dark background, is found in Australia. It is quite rare, and large pieces such as the ones shown here have become extremely valuable. (Almost actual size.)

Opal is unlike most gemstones in that its flashing color is not due to the color of the stone itself, or even to the color of its included impurities. Rather, it is due to the way in which tiny opal particles are grouped during its formation. Detailed photographs taken through an electron microscope show clearly how precious opal is deposited as spheres that are so small they are indistinguishable under powerful optical microscopes. These spheres are packed together in very orderly networks, row upon row and layer upon layer, with tiny open spaces, also in rows, between them. Masses of common opal lack this orderly internal arrangement of spheres. White light striking the precious opal is reflected independently by each row of spheres, much like the reflections from a series of slats in a venetian blind.Since these rows of spheres are spaced at distances approximately the same as the wavelength of light, a phenomenon known asdiffractionoccurs. The separate reflections interfere with each other in an organized manner, cancelling out some of the light wavelengths and reinforcing others, producing color. The brilliant color flashes are of different hues depending on the sizes of the spheres of opal and, therefore, the distances between rows. To provide the best display of this optical effect, opal is almost always cut in cabochon form rather than as faceted stones.

Fire opals have rich fire; some have background colors that vary from bright yellow through orange and red; and some are colorless. Stones such as the ones shown here, which weigh 7, 11, and 22 carats, have made Querétaro, Mexico, famous as their source. (Actual size.)

This rare 34-carat opal from Brazil resembles closely the opals found in Australia. (Actual size.)

Common opal, which shows milky opalescence, does not exhibit color flashes, and it is not used as a gemstone. Each of the common varieties—such as hyalite, cacholong, and hydrophane—has its own slightly different set of characteristics, but only precious opal, with its dazzling color display, is important for gem purposes. To take full advantage of the small amounts of gem material available, or to bring out its color better,preciousopal is often cut as thin pieces and mounted as doublets on some other backing. Also, the seams in rock sometimes are cut so that the thin layer is exposed on athicker backing of the adjoining rock. Precious opal, or gem opal, is classified aswhite opalwhen the color flashes are in a whitish or light background,black opalwhen the background material is gray, blue-gray, or black, andfire opalwhen the background is more translucent and red, reddish orange, or reddish yellow.

Precious opal has been found in several areas of the world—in nodules, in seams in rock, or as replacements of other minerals or even of wood and shell. Hungarian deposits were well known in Roman times, but these and other deposits became insignificant with the discovery of opal in Australia in the late 19th century. Opal deposits were discovered in 1889 at White Cliffs in New South Wales, and other important discoveries in Australia followed, including deposits at Lightning Ridge in New South Wales that produce very dark stones and the rich fields of white opal at Coober Pedy in South Australia. Mexico has remained for a long time the principal source of richly colored fire opals, with the most important deposits located in the state of Querétaro, where mines have been worked intermittently since 1835. This has made the town of Querétaro today the center for the trade and cutting of Mexican opal.

Two of the more famous stones in the British crown jewels are the Black Prince’s Ruby and the Timur Ruby, but neither of these stones is really ruby. Like the great red gem in the crown that belonged to the Russian Empress Catherine II, these two British stones are spinel. Although spinel occurs in many colors, such as yellow, green, violet, brown, and black, it is the red spinel that usually is seen in thegem trade. There are several varieties of red spinel, such asruby spinel,balas ruby,rubicelle, andalmandine spinel—all of which refer to the color resemblance to ruby.

The hues and tints of spinel show subtle variations that are matched only by those of tourmaline. Unlike tourmaline, however, spinel may be bright ruby red. The cut stones curving around two pieces of rough from Burma weigh (left to right) 30 carats (Ceylon), 34 carats (Burma), 36 carats (Burma), 30 carats (Ceylon), and 22 carats (Ceylon). (Three-fourths actual size.)

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.

Synthetic blue spinel is widely used as a substitute for aquamarine, and synthetic spinels of other colors are used as substitutes for many gems. However, the synthetic stones are not ordinarily made in the subtle shades so characteristic of natural spinel. Completely colorless spinel apparently exists only as a synthetic material. Actually, because of its hardness, durability, and many attractive colors, spinel makes a fine gemstone in its own right.

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.

Few gemstones can compete with quartz for variety of color. Having a hardness of 7 and occurring in many beautiful varieties, only the relative abundance of quartz prevents the species from attaining top rank among gemstones.

The two kinds of quartz, crystalline and cryptocrystalline (fine-grained) quartz, occur in all kinds of mineral deposits throughout the world. Much of this material is suitable for cutting gems.

Colorless crystalline quartz, orrock crystal, makes attractive faceted gems, and it is used as a suitable substitute for diamond and zircon even though it lacks the fire and brilliance of those gemstones. Some very large, flawless crystals of colorless crystalline quartz have been found. The great Warner Crystal Ball, with a diameter of 12⅞ inches and weighing 106¾ pounds, was cut from such a crystal. In addition to the name rock crystal, colorless crystalline quartz appears in the jewelry trade under such names as rhinestone (not to be confused with the glass substitute), Herkimer diamond (fromHerkimer County, N. Y.), and Cape May diamond (from Cape May, N. J.).

The most popular variety of quartz isamethyst, a transparent form whose color ranges from pale violet to deep purple. In many cut stones of amethyst the color intensity changes sharply from section to section. This is due to irregular color zoning common to amethyst crystals. The actual cause of the purple color in amethyst is not very well understood. There are fewer cut stones of amethyst in very large sizes because of the rarity of large, flawless, well-colored crystals.

This 4500-carat pale smoky quartz egg from California rests on a gold stand set with Montana sapphires. The unique gem was cut and its stand was designed and made by Capt. John Sinkankas as a difficult exercise in the lapidary art. The quartz egg is 4 inches long and almost 3 inches in diameter.

The namecitrine(from the French word for lemon) attempts to describe the yellow color of another variety of quartz. Actually, the normal coloring of citrine varies from yellow to red-orange and red-brown, but the yellow sometimes rivals the yellow of topaz. In addition to the normal color range, the colors of citrine may grade through a grayish yellow variety known ascairngormand a grayish variety calledsmoky quartzto a black variety calledmorion. Other varieties that add color dimensions to the group of quartzgemstones arerose quartzandmilky quartz. Like amethyst, the reason for the color in rose quartz has not been definitely established. Milky quartz owes its color to myriads of tiny cavities containing water or liquid carbon dioxide.

A 783-carat step-cut citrine of deep, rich color dwarfs a 278-carat brilliant-cut citrine (at left), a 90-carat smoky quartz, and a 91-carat briolette of citrine. The smoky quartz, from Switzerland, is so dark that it appears to be opaque. The other three stones came from Brazil. The briolette and brilliant-cut citrines were cut and donated to the Smithsonian Institution by Albert R. Cutter. (Slightly less than half actual size.)

The range of color in quartz is somewhat surprising, considering that the mineral is a simple silicon dioxide. Some of the colors, as with corundum and some other gemstones, are due to traces of impurities. In quartz, these consist mainly of oxides of iron, manganese, and titanium. However, all the reasons for quartz coloration in its many varieties are not known.

Pastel rose quartz has a delicate beauty in any cut. The 375-carat step cut (top), the 84-carat step cut, and the 46-carat marquise came from Brazil. (Two-thirds actual size.)

Amethyst, a purplish quartz, is the birthstone for February. Here it is represented by a 1362-carat stone from Brazil (top), a 54-carat stone from Pennsylvania (left), and a 21-carat stone from North Carolina. (Almost actual size.)

In addition to possessing wide variation of color, quartz, like sapphire and certain other gemstones, can exhibit asterism or chatoyancy. The well-knowntiger’s-eyefrom West Griqualand, South Africa, owes its eye effect to the fact that its material is a replacement of fibrous asbestos by cryptocrystalline quartz. 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 inclusionsis 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.

If the foreign inclusions consist of tiny flakes of hematite or mica, the quartz assumes a spangled appearance and is calledaventurine.

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.

Although quartz occurs in many varieties and its crystals are cut in many styles, it is easily identified by its refractive index of 1.55, specific gravity of 2.65, and hardness of 7.

With color ranging from shades of yellow and brown through blue-green to olive, and with a hardness of 8½, chrysoberyl has most of the characteristics necessary for a fine gem. Rare stones of high-quality chrysoberyl demand fairly high prices, and they are sought eagerly by the connoisseur of gemstones.

Chrysoberyl is beryllium aluminate, and thus is closely related to the gemstone spinel, which is magnesium aluminate. When pure, chrysoberyl is colorless and relatively uninteresting as a gemstone because of its lack of color dispersion and its moderate refractive index of 1.75. However, few pure samples are known, as chrysoberyl normally contains some iron or chromium in place of aluminum and some iron in place of beryllium. As a result of such impurities, the color of chrysoberyl my be yellowish, greenish, or brownish.

Chrysoberyl and beryl are the only important gemstones containing the element beryllium. The minerals beryllonite, euclase, hambergite, and phenakite also contain this element, but they are rare and seldom are seen as cut gems.

One of the finest chrysoberyl cat’s-eyes in existence is the 58-carat Maharani from Ceylon. (Actual size.)

Thealexandritevariety of chrysoberyl has two colors in delicate balance, and it changes from a columbine red to an emerald green when viewed under different light. When viewed in daylight, which is richer in green, the color balance shifts toward green, and that hue is seen by the observer. Under artificial light, normally richer in red, the color balance shifts toward red, and the stone seems to have changed to that color.This extremely rare stone, named after Czar Alexander II of Russia, is found only occasionally, in Russia and Ceylon. The Russian stones, found with emerald in mica schist, tend to be smaller than the Ceylon stones and have a color change going from emerald green to violet-red. The Ceylon stones, found as pebbles in gem gravels, have a color change going from a less-emerald green to a browner red. The 66-carat, record-size alexandrite in the National Collection shows the color change typical of Ceylon stones. A synthetic stone is commonly marketed as synthetic alexandrite, but this substitute not only is man-made but is actually synthetic corundum instead of synthetic chrysoberyl.

In addition to its fine cat’s-eyes and its color-changing alexandrite varieties, chrysoberyl occurs in handsome stones that vary in depth of color. Shown here with an uncut twinned crystal of gem quality from Brazil are a 46-carat stone from Brazil (left) and a 121-carat stone from Ceylon. The uncut crystal is a gift of Bernard T. Rocca, Sr. (Two-thirds actual size.)

Cat’s-eyechrysoberyl contains myriads of tiny fiberlike channels arranged in parallel position. When the stone is cut as a cabochon, a band of light is reflected from the curved top of the stone, producing an effect that resembles the slit pupil of a cat’s eye.

Because of its great color range, which includes pink, green, blue, yellow, brown, and black in many different shades and combinations of shades, tourmaline is one of the most popular of the colored gemstones. Tourmaline with a color near emerald green is particularly popular.

Chemically, tourmaline is a very complex borosilicate, and its color is determined by the various elements present in it. Tourmaline crystals having sodium, lithium, or potassium are either colorless, red, or green; those having iron are blue, blue-green, or black; and those having magnesium are colorless, yellow-brown, or blackish brown.

Some crystals of tourmaline are of two colors, and stones of mixed colors, such as pink and green, can be cut from these. The color mixing may show as zoning with the core color of the crystal overlaid by another color and perhaps even additional layers of other colors. Zoned crystals with a core of deep pink covered by a layer of green have been called “watermelon tourmaline.” Because its refractive index of about 1.6 is too low to give it marked brilliance, and its color dispersion is too low to give it fire, the tourmaline relies almost solely on the beauty of its color for its rank in popularity.

Although tourmaline has a low refractive index and low dispersion, it exhibits remarkable dichroism. In other words, it can present different tints to the viewer depending on the direction that the light is traveling through the crystal. When viewed down the long, or vertical, axis of the crystal, the color of tourmaline is much stronger than when viewed from the side. This means that if the crystal is dark the cutter will have to cut the stone with the flat part, or table, parallel to the long axis of the crystal. The color of the gemstone then will be lightened when viewed from its table, since this is the direction of lighter color. Similarly, the table of a lighter colored crystal can be cut perpendicular to the long axis in order to produce a deeper colored gem.

Green seems to be the best known commercial color of tourmaline, but this extremely variable gem species exhibits many subtle shades of color, as shown here. At upper left, a 104-carat stone from Mozambique; at upper right, a 173-carat stone from Mozambique; at lower left, a 111-carat stone from Manchuria; and a 35-carat stone from Brazil. (Actual size.)

Some tourmaline crystals contain threadlike tubes or inclusions of microscopic size running parallel to its length. When cut as cabochons, such crystals give a good “cat’s-eye” effect.

Tourmaline has no distinct cleavage and has a hardness somewhat above 7, and these characteristics make the stone sufficiently resistant to normal shock and wear so that it is highly satisfactory for use in jewelry.

Noted deposits of tourmaline are located in the Ural Mountains of Russia, Ceylon, Burma, South-West Africa, Madagascar, Brazil, Maine, and California. Crystals from each of these localities seem to have their own color specialties. The deposits in San Diego County, Calif., are unique in that all colors except brown are found there. In the early 1900’spink and red tourmaline was shipped from there to China for carving, but this thriving trade stopped with the end of Chinese imperial reign. The tourmaline deposits at Paris, Auburn, and Hebron, Maine, have furnished a number of excellent gems, especially of blue and green colors.

Zircon, because of its high refractive index and high dispersion, approaches diamond in degree of brilliance and fire. On only casual examination it is quite possible to mistake a well-cut, colorless zircon for a diamond. However, a careful examination of the back facets of such a stone, when viewed through the table, would show strong double refraction, a characteristic of zircon but not of diamond. Zircon’s double refraction makes the back facet edges appear doubled. Since diamond is “singly refracting,” it cannot produce this double appearance of the back facets.

Zircon is brittle and has a hardness of just over 7, while diamond’s hardness, as we have seen, is rated at 10. After being worn in jewelry for a long period of time, zircon will show signs of chipping on the facet edges. Under the same conditions, diamond would remain unchanged. Because of this tendency for facet edges to chip, it is the practice in the gem trade to pack cut zircons separately. If a number of zircons were placed in the same paper packet there would be a risk of “paper wear.”

In the gem trade, the most important zircons are those that are colorless, golden brown, or sky blue. Such stones originally were reddish brown zircon pebbles from Indochina, but they have been converted by being subjected to temperaturesapproaching 1800° F. for periods of up to two hours. When the original zircons are heated in a closed container, the stones become blue or colorless; when a flow of air is allowed through the container, the stones become golden yellow, red, or colorless. In most of these converted stones the color remains quite stable, but in some it may revert to an unattractive greenish or brownish blue after a period of time.

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