Part 3

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.

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 word _oriental_ often 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.

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.

CRYSTAL AXIS POSITION STONE MUST TAKE TO SHOW STAR OTHER STAR STONES MAY BE CUT, BUT MUST BE IN THE SAME POSITION WITHIN THE CRYSTAL ROUGH SAPPHIRE CRYSTAL CRYSTAL AXIS

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.

VARIETIES Ruby: Red. Sapphire: Blue, yellow, pink, green, colorless, and any color except red. Star sapphire: Colored as sapphire and showing asterism. Star ruby: Red and showing asterism.

BERYL (INCLUDES EMERALD AND AQUAMARINE)

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 as _emerald_, and when it is the blue-green of sea water it is called _aquamarine_. Varieties such as the rose-pink _morganite_, golden-yellow _heliodor_, and colorless _goshenite_ are 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.

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, which produces 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.

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 a 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.

VARIETIES Emerald: Grass green Aquamarine: Blue green Morganite: Pink Heliodor: Yellow Goshenite: Colorless

TOPAZ

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.

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

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.

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 as _diffraction_ occurs. 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.

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, _precious_ opal 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 a 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.

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.

VARIETIES White opal: Color flashes in light-colored background material Black opal: Color flashes in dark gray or bluish background material Fire opal: Orange or reddish background material

SPINEL

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 the gem trade. There are several varieties of red spinel, such as _ruby spinel_, _balas ruby_, _rubicelle_, and _almandine spinel_—all of which refer to the color resemblance to ruby.

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.

VARIETIES Almandine spinel: Purplish red Rubicelle: Orange-red Balas ruby: Rose red Ruby spinel: Deep red Chlorospinel: Translucent grass green Ceylonite or pleonaste: Opaque dark green, brown, or black Picotite or chrome spinel: Translucent dark yellow-brown or green-brown

QUARTZ (INCLUDES ROCK CRYSTAL, AMETHYST, AND CITRINE)

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, or _rock 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 (from Herkimer County, N. Y.), and Cape May diamond (from Cape May, N. J.).

The most popular variety of quartz is _amethyst_, 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.

The name _citrine_ (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 as _cairngorm_ and a grayish variety called _smoky quartz_ to a black variety called _morion_. Other varieties that add color dimensions to the group of quartz gemstones are _rose quartz_ and _milky 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.

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.