Part 2

Shaping of gemstones is not limited to geometric faceting. Many stones, especially those which are opaque or which produce stars and cat’s-eyes, are cut as _cabochons_. This ancient, and probably oldest, cutting style consists merely of a raised and rounded form. When extended completely around the stone, the cabochon form results in a bead that can be drilled and strung. Many cabochons, especially those of less expensive gem materials, are now cut in large quantities to standard sizes in order to fit mass-produced gem mountings.

Sculpting in gemstones is a much more intricate, nongeometric kind of shaping. Although tools differ in detail, and the gem sculptor must possess an artistic eye as well as lapidary skill, the basic processes of sawing, grinding, and polishing are the same.

4 GEM SUBSTITUTES

Because of their rarity and relatively high cost, the number of real gems used throughout recorded times must be insignificant compared to the number of gem substitutes used. 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.

In general, gem substitutes can be classified as imitation stones, assembled stones, reconstructed and altered stones, and synthetic stones.

IMITATION STONES

Any material will serve as an imitation of a natural gem as long as it resembles the real thing under casual examination. Because of the great variety in types and colors available, glass and plastics are the most commonly used materials for making imitation gems. Almost every gem has been simulated effectively. The substitutes offer no difficulty of identification to the expert, but many are deceptive to the layman.

ASSEMBLED STONES

It has been the practice for centuries to build up gemstones by fusing or cementing a shaped piece of natural gemstone to another piece, or other pieces, of inferior or artificial material.

A colorless common beryl crown cemented to a pavilion of green glass produces an emerald doublet—part natural, part artificial—of good color and high durability. A thin piece of beautifully colored opal cemented to a base of inferior opal provides an assembled stone that looks like a thick piece of high-quality opal. Triplets, and even stones in which there are pockets of colored liquids or metal foil between the shaped pieces, are known.

Usually, assembled stones are easily detected, since the joint will show under magnification, but sometimes they are mounted in settings that obscure the joint, and detection is more difficult.

RECONSTRUCTED AND ALTERED STONES

Ruby fragments may be heated at high temperature to partially melt them into a large mass that can be cut into a more valuable stone. Ruby is the only stone that can be successfully reconstituted in this way, but there are many other ways of tampering with natural stones to make them more desirable.

Sometimes natural stones are backed with foil or a metallic coating to enhance their color, to provide brilliance, or to produce a star effect. It is said that in an inventory of the Russian crown jewels by the Soviet Government, the ruby-colored Paul the First Diamond was discovered to be a pale pink diamond backed by red foil. Today, some diamonds are coated on the back with a blue film to improve their color.

Aquamarine, when pale greenish blue, may be heated in order to deepen the blue color, and poorly colored amethyst may be heated to produce a beautiful yellow-brown quartz, called citrine, that often is misrepresented as topaz. By strong heating, the brown and reddish brown colors of zircon can be changed to blue or colorless, both of which states are unknown in natural zircon. Dyes, plastics, and oils are used to impregnate porous gems such as turquoise and variscite, and even jade. Off-color diamonds, when exposed to strong atomic radiation, can be changed to attractive green, brown, and yellow colors, causing them to resemble higher-priced _fancies_.

In the constant search for something new, gem suppliers sometimes introduce into gemstones colors that are not always an improvement. For example, the beautiful purple of some amethyst can be converted, by heat treatment, to a peculiar green. Such an altered stone is marketed as _greened amethyst_.

All of this tampering with gemstones complicates the problem of identification, so it is a matter of serious concern to the gem trade.

SYNTHETIC STONES

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.

Two of the basic techniques used in producing synthetic gems are the _flame-fusion_ and the _hydrothermal_ processes.

In the flame-fusion process—invented in 1904 by the French chemist Verneuil—powdered aluminum oxide, containing coloring agents, is sieved down through the flame of a vertical blowtorch furnace. As it passes through the flame, the powder melts and accumulates as drops on an adjustable stand just below the flame, where it forms a single crystal _boule_ of the synthetic rough. In a few hours a boule of several hundred carats can be formed. When such furnaces are operated in banks of several hundred units, the commercial production of corundum alone becomes possible at the rate of many tons a year. Through the years, of course, refinements have been made on Verneuil’s original furnace.

In the hydrothermal process, which differs greatly from Verneuil’s flame-fusion process, crystals are grown from solutions of the raw materials that have been subjected to varying conditions of very high pressure and temperature. Some of the quartz used for electronics purposes also is manufactured in this way.

Since chemical composition and crystal structure are the basic characteristics by which a gemstone is identified, and these characteristics are identical in both the manufactured stone and its natural counterpart, the synthetic gemstones offer a very serious challenge to those concerned with gem identification.

5 GEM LORE

All sorts of magic and symbolic properties have been ascribed to gemstones through the ages; for example, the cat’s-eye has been prescribed as a cure for paleness, citrine has been worn as a protection from danger, and the opal cherished as the symbol of hope. The result has been the creation of an intricate, chaotic, and contradictory but interesting mass of gem lore.

Among the treasures in the Smithsonian’s Museum of Natural History is a very old silver breastplate that once was in an ancient synagogue and supposedly was modeled after the one worn by Aaron, the first high priest of the Hebrews. In this plate are mounted twelve stones representing the Twelve Tribes of Israel. Among Christians, the Twelve Apostles also were represented symbolically by precious stones.

THE TWELVE TRIBES Levi, _Garnet_ Zebulon, _Diamond_ Gad, _Amethyst_ Benjamin, _Jasper_ Simeon, _Chrysolite_ Issachar, _Sapphire_ Naphtali, _Agate_ Joseph, _Onyx_ Reuben, _Sard_ Judah, _Emerald_ Dan, _Topaz_ Asher, _Beryl_

THE TWELVE APOSTLES Peter, _Jasper_ Andrew, _Sapphire_ James, _Chalcedony_ John, _Emerald_ Philip, _Sardonyx_ Bartholomew, _Sard_ Matthew, _Chrysolite_ Thomas, _Beryl_ James the Less, _Topaz_ Jude, _Chrysoprase_ Simon, _Hyacinth_ Judas, _Amethyst_

The number “12” seems to follow a chain of gemstone superstitions. Gemstones were considered to have mystical relationship not only with the Twelve Tribes and the Twelve Apostles but also with the Twelve Angels, the Twelve Ranks of the Devil, and the Twelve Parts of the human body.

Some stones were even endowed with astrological significance and were believed to be in sympathy with the twelve zodiacal signs. On the basis of an elaborate system of prognostications, an astrologer was considered able to foretell future events by proper observance of changes in hue and brilliance of the symbolic stones.

Aries the Ram, _Bloodstone_ Taurus the Bull, _Sapphire_ Gemini the Twins, _Agate_ Cancer the Crab, _Emerald_ Leo the Lion, _Onyx_ Virgo the Virgin, _Carnelian_ Libra the Scales, _Chrysolite_ Scorpio the Scorpion, _Aquamarine_ Sagittarius the Archer, _Topaz_ Capricornus the Goat, _Ruby_ Aquarius the Water Bearer, _Garnet_ Pisces the Fishes, _Amethyst_

Perhaps in our own space-oriented times the ancient superstitions sympathetically relating certain gemstones with the planets will be revived. 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. Many birthstone lists have been proposed, and in order to use this idea to popularize gemstones the American jewelry industry has agreed upon an official list. This list has served to bring about some uniformity in the selection of birthstones for the twelve months.

January, _Garnet_ February, _Amethyst_ March, _Aquamarine_ or _Bloodstone_ April, _Diamond_ May, _Emerald_ June, _Moonstone_ or _Pearl_ July, _Ruby_ August, _Peridot_ or _Sardonyx_ September, _Sapphire_ October, Opal or _Tourmaline_ November, _Topaz_ or _Citrine_ December, _Turquoise_ or _Lapis lazuli_

All these associations and strange beliefs have served to create in the general public a mental image of gemstones that gives to them an increased exoticism and mysterious appeal far exceeding their monetary value.

6 PRINCIPAL GEM SPECIES

An excursion into the literature of gems would reveal that there is much to be discovered about them other than the cold facts of gemology, techniques of gem cutting, and tales of gem lore. When all the information about an individual species is assembled, it provides a sketch of a fascinating gemstone personality. Whole books have been written about diamond—books filled with essays on its mining history, natural occurrences, scientific significance, and best known cut stones.

In the following sections of this book, some of the facts about several of the better known gem species have been gathered. The treatment is not meant to be complete, but enough information is given so that the Museum visitor may better understand and remember what he has seen.

For each species described there are color illustrations of certain gemstones displayed in the collection. Several photographic and artistic techniques have been used to emphasize the various aspects of the beauty of these stones, many of which are the largest and finest of their kinds known; however, not all of the finest gems are pictured here.

At the end of this descriptive section is a list of the significant faceted gemstones in the collection. Obviously, this list will change, because new gemstones constantly are being acquired.

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 its _grain_.

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 Great Mogul, 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.

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.

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

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 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 oysters 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 most _blister 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, called _baroques_, 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, called _oriental 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-water pearls 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 growing _cultured pearls_ has 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.

CORUNDUM (RUBY AND SAPPHIRE)

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.

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.