76 A Book of Precious Stones
While
individual pearls or strands of them may be worth a prince's ransom,
their beauty and value are not immutable; pearls may deteriorate with
age or be sullied by the action of gases, vapours, or acids, and the
known methods for their restoration to their original appearance and
value are not always successful. Fine pearls should be carefully wiped
with a clean soft cloth after they have been worn or exposed, and kept
wrapped in a similar fabric in a tightly closed casket.
Pearls
are found in nearly all bivalves with nacreous shells, but the
principal supply is derived from a comparatively few families, led by
the Aviculidse, Unionidse, and Mytilidse. The first group includes the
pearl oyster of the Indian and Pacific oceans, from which has come the
bulk of the world's pearls; the second includes the unio, or
fresh-water mussel of North America; and the third is a family of
conchi-ferous molluscs, mostly marine, the typical gems being Mytilius edulis, or
true mussel, which has a wedge-shaped cell and moors itself to piles
and stones by a strong coarse byssus of flaxy or silky-looking fibres.
The distribution of these molluscs is world-wide.
" In all ages, pearls have been the social
