PEARL, a composite of
Aragonite and conchiolin
often considered a gemstone,
although it is not a mineral. Rather, pearl is a composite of the mineral
(Calcium Carbonate) and the organic compound conchiolin (a protein). The
combination is called nacre, or mother-of-pearl. In some cases, there is an
admixture of the mineral
calcite (another mineral composed of Calcium Carbonate). In pearl
oysters and freshwater pearl mussels, nacre forms the inner lining of the
shell. In most other molluscs the shell has an appearance more like
porcelain, and lacks the luster and iridescence of mother-of-pearl.
A pearl is formed inside these molluscs when minute hexagonal crystals of
aragonite and conchiolin (as a binder) are deposited over an irritant. As a
composite, it is tougher than the aragonite from which it's formed, and the
layered nature of the structure contributes to the luster of the pearl.
An ideal pearl is spherical, but other shapes are common. A pearl may
even become cemented against the inner wall of the shell, and eventually
become a blister pearl.
Most pearls are "cultured", meaning the oysters (or mussels) have been
opened and a "seed" inserted into the flesh of the oyster. The oyster then
secretes nacre over the seed, eventually covering it with mother-of-pearl
and a pearl is formed. Note that a natural pearl of significant size takes
many years to form, as only a fraction of a millimeter of nacre is deposited
each year. But for a cultured pearl, a bead is inserted as a seed (typically
composed of shell), and the oyster may be harvested (and the pearl removed)
in as little as six months, although for higher quality pearls (a thicker
layer of mother-of-pearl) the harvest may not be done for several years.
Different species have different secretion rates of nacre, so the minimum
time varies widely.
At one time, pearls were very rare and expensive, because more than a
thousand oysters might be harvested to find a single pearl, and that one may
have been small or misshapen. However, raising oysters or freshwater pearl
mussels is relatively easy, and the process of inserting the "seeds" has
been perfected, such that cultured pearls are now harvested by the ton.
Still, only a fraction of the harvest will be of high-quality pearls.
Note that imitation pearls are common. These may be made of ceramics,
plastics, or other materials. The simplest test (which is not,
unfortunately, infallible) is the "tooth" test. Rub the pearl against the
front of your tooth. If it feels smooth, it is a fake pearl. If it feels
slightly gritty, it may be genuine (or a really well-made fake). There is no
easy test for natural versus cultured pearls, other than to break it or
slice it to see if the interior is all concentric layers (as in a natural
pearl) or if the center (the bulk of the pearl) is ordinary shell material
Pearl is a Birthstone for June,
and for the astrological signs of
Scorpio and Cancer. It is the traditional gift for a 30th
anniversary (reflecting its historical
high value), although in the modern lists Pearls are to be given for
the twelfth anniversary. Pearls are
associated with the values of modesty, purity, beauty, and happiness.
sells natural mineral specimens, including precious metals and gemstones, but not (at present) Pearls.
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