HIS
beautiful gem, formed by nature in the shells of oysters and of
mussels, is found in the beds of rivers and in the sea, in various
places in Europe, Asia, and America.
Its
chemical composition is entirely carbonate of lime and organic matter.
It possesses a lustre peculiar to itself, which is called pearly : it
is easily affected by acids and fetid gases, and calcines on exposure
to heat. The specific gravity is 2.5 to 2.7 ; those found on the coast
of South America, termed Panama pearls, are somewhat heavier than the
oriental ones.
Naturalists
account for the formation of these gems in the endeavours of the animal
to rid itself of some foreign body which has intruded into its shell,
by covering it with a deposit similar to the interior of the oyster
