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 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, 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.
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 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.
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.
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.
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, 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.
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 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