populated
and the streams so contaminated with sewerage and refuse from
factories, that animal life is rapidly disappearing from the
water-courses in many localities.
It
is probable that the existence of carbonate of lime in excess where
mussels abound influences the secretion that causes the growth of the
pearl. In limestone regions, where the waters are polluted by products
of decompositions that are acid, these unite with the lime and form
other compounds, that are precipitated or are carried away with the
impurities of the water. There can be no doubt that this cause would
tend to decrease the amount of lime which the shell would receive, thus
not only retarding the growth of the pearl, but often eventually
leading to the extermination of the Unio itself. At nearly all the
marine pearl fisheries, coral-banks abound, and it may be that these
have more or less influence on the development of the pearl in the
shell. In Vermont, New Jersey, and Ohio, where pearls, were formerly
found, a fine one is now rarely obtained. In gathering the shells, only
those that are full-grown, old, and distorted by disease should be
taken, so that the fisheries may be preserved, and the shells should be
opened as soon as taken from the water, and not allowed to open by
decay for this discolors the pearls; and particularly, they must never
be opened by boiling, as this dims the lustre and lessens the value of
the pearl.
The
common clam (Venus mercenaria) secretes pure white pearls, scarcely
distinguishable from ivory buttons, as well as others faintly tinted
with a purplish blue, passing at times to a reddish purple and a
purplish black. The white pearls are worthless, the tinted ones of very
little value, but those of darker color are often from 1/4 to 3/8 of an
inch in diameter, and the finest ones bring from $20 to $100. The
supply is limited and there is very little demand, for unless the color
is exceptionally good they possess little beauty, lacking the lustre
peculiar to other pearls ; still, when mounted with diamonds, the
appearance of the darker ones is much improved.
It
would seem from an article on wampum, written by Dr. Samuel L. Mitchell
in 1825, that clam pearls were of much more frequent occurrence in the
early part of the century than they
