sm. There are two principal types of molluscs which yield
true pearls in commercial quantities. The best known of the first type
is the so-called pearl oyster (_Meleagrina margaritifera_). The pearl
mussel of fresh water streams is of the second type (_Unio
margaritifera_). Other species of molluscs having pearly linings to
their shells may produce pearls, but most of the pearls of commerce come
from one or the other of the two varieties mentioned.

STRUCTURE OF PEARL. The structure and material of the true pearl must be
first understood in order to understand the underlying reasons for the
remarkable beauty of this gem. Pearls are composed partly of the mineral
substance calcium carbonate (chemically the same as marble) and partly
of a tough, horny substance of organic nature called conchiolin. The
shell of the pearl-bearing mollusc is also composed of these two
substances. Calcium carbonate may crystallize in either of two forms,
calcite or aragonite. In marble we have calcite. In the outer portions
of the shell of the pearl oyster the calcium carbonate is in the form of
calcite, but in the inner nacreous lining and in the pearl itself the
mineral is present as aragonite. This is deposited by the mollusc in
very thin crystalline layers in the horny layers of conchiolin, so that
the lining of the shell is built of approximately parallel layers of
mineral and of animal substance. In the normal shell this is all that
takes place, but in the case of a mollusc whose interior is invaded by
any small source of irritation, such as a borer, or a grain of sand, or
other bit of foreign material, a process of alternate deposit of
conchiolin and of aragonite goes on upon the invading matter, thus
forming a pearl.

The pearl is built in layers like an onion. In shape it may be
spherical, or pear-shaped, or button-shaped or of any less regular shape
than these. The regular shapes are more highly valued. The spherical
shape is of greatest value, other things being equal. Next comes the
drop or pear shape, then the button shape, and after these the host of
irregular shapes known to the jeweler as "baroques." The river man who
gathers mussels calls these odd-shaped pearls "slugs."

Let us now attempt to understand how the beautiful luster and
iridescence of the pearl are related to the layer-like structure of the
gem. In the first place, it should be understood that both conchiolin
and aragonite are translucent, that is, they pass li