pearl
, concretion formed by a
mollusk
consisting of the same material (called nacre or
mother-of-pearl
) as the mollusk’s shell. It is a highly
valued
gemstone
. Pearls are often strung into a necklace after a small hole is drilled by hand-driven or electric
tools
through the centre of each pearl (
see also
jewelry
).
Pearls are characterized by their translucence and
lustre
and by a delicate play of surface colour called
orient
. The more perfect a pearl’s shape (spherical or droplike) and the deeper its lustre, the greater its value. Only those pearls produced by mollusks whose shells are lined with mother-of-pearl (e.g., certain species of both saltwater
oysters
and freshwater
clams
) are really fine pearls; pearls from other mollusks are reddish or whitish, like
porcelain
, or lacking in pearly lustre. Jewelers commonly refer to
saltwater pearls as Oriental pearls and to those produced by freshwater mollusks as
freshwater pearls.
Britannica Quiz
(Bed) Rocks and (Flint) Stones
The chief component of the nacre that
constitutes
the pearl is
aragonite
(CaCO
3
). Nacre also contains a small amount of conchiolin, a hornlike organic substance (albuminoid) that is the main
constituent
of the mollusk’s outer shell. The shell-secreting cells of the mollusk are located in the
mantle
, or
epithelium
, of its body. When a foreign particle penetrates the mantle, the cells attach to the particle and build up more or less concentric layers of pearl around it. Irregularly shaped pearls are called
baroque pearls
. Pearls that grow attached to the shell are often flat on one side and are called
blister pearls
.
The colour of pearls varies with the mollusk and its
environment
. It ranges from black to white, with the rose of Indian pearls esteemed most. Other colours are cream, gray, blue, yellow, lavender, green, and
mauve
. All occur in delicate shades. The surface of a pearl is rough to the touch. Pearls come in a wide range of sizes. Those weighing less than
1
/
4
grain (1 pearl grain = 50 mg =
1
/
4
carat) are called
seed pearls. The largest naturally occurring pearls are the baroque pearls; one such pearl is known to have weighed 1,860 grains (121 grams [about 4.3 ounces]).
The finest Oriental pearls are produced by the
mohar
, the Atlantic pearl
oyster
(
Pinctada imbricata
). Found in the
Persian Gulf
, with the richest harvest taken from the waters off the great bight that curves from the peninsula of Oman to that of Qatar, the pearls come from depths of 8 to 20 fathoms (14.6 to 36.6 metres [48 to 120 feet]). Other notable sources of fine-quality pearls include the
Gulf of Mannar
between India and Sri Lanka; the waters off Celebes, Indonesia; and the islands of the South Pacific. In the Americas the
Gulf of California
, the
Gulf of Mexico
, and the Pacific coast of Mexico have yielded dark-hued pearls with a metallic sheen as well as white pearls of good quality.
Freshwater
mussels
in the temperate zone of the Northern Hemisphere have produced pearls of great value?for example, those from the
Mississippi River
. Pearling is a carefully fostered industry in central Europe, and the forest streams of Bavaria, in particular, are the source of choice pearls. Freshwater pearling in
China
has been known from before 1000
bce
. In all pearl fisheries, however, production has declined significantly since the widespread introduction of
cultured
pearls (
see
cultured pearl
).
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