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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.)]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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