forms
being so great, an artist has a wide field for imagination. The
pearls,, however, have but slight value unless they are beautiful and
lustrous.
Frequently
pearls have an opaque appearance and seem to be worthless, but on the
removal of their outer layer are found to be clear and iridescent. This
outer layer may be removed by dipping them in a weak solution of acid,
which dissolves the opaque coating, or it may be peeled with a knife,
although sometimes the pearl is not of the same material throughout,
and cannot be restored. The story is told of a New York lady who
purchased a button-shaped Unio pearl that had a black, diseased
appearance on one side. It was so set that the imperfection was all
below the mounting. When applauding at the opera one evening, the pearl
was broken, and on examination it was found to consist of a very thin
nacreous layer, inside of which was nothing but a hard, white, greasy
clay. (See Illustration.) Whatever be the method of their formation, it
would seem that pearls can be formed only at the expense of the shell,
for every substance necessary to their growth is drawn from sources
which normally secrete the shell. Hence the presence of the pearl can
usually be detected on the outside of the shell. Normal appearing
shells rarely contain pearls, while on the other hand those that are
deformed often contain pearls of great beauty. There are three
indications on which pearl-fishers rely for detecting from the outward
aspect of the shell the presence of pearls. These are, first, the
thread, that is, the recess or elevation extending from the vertex to
the edge; second, the kidney-shape of the shell, that is, an
indentation on the ventral side ; and third, the contortion of both
shells toward the middle plane of the animal.
Much
interesting information concerning the structure and quality of the
shells of fresh-water pearl-bearing mussels was obtained at the
International Exhibition held at Berlin in 1880. The shells were found
to consist of three strata: first, the outer yellow or brown
conchioline (cuticula or epidermis); second, the prism stratum,
consisting of layers formed of minute prisms arranged vertically to the
layers and the shell surface ; and third, the interior nacre layer,
composed of finely folded leaves
