LXXV1I.
PEARL.
Under this
name is understood in jewellery a conÂcretion of carbonate of lime,
round or tubercular, conÂtaining some organic substance.
It has a light peculiar to itself, and therefore called pearly. It is generally of a milky-white colour, but is sometimes green, red, blue, and yellow.
Its
specific gravity is from 2 684; in consistency it equals all other
calcareous substances, and, under the action of a diluted acid, it
changes into a gelatinous substance, but only on the surface, the inner
parts remaining intact. It is formed of various pellicles, one over the
other, in a similar manner to the onion.
The pearl is not a gem, but simply the production of a secretion of some bivalve shell.
