212 GEMS AND PRECIOUS STONES IN THE
process
of their production, these interior concretions may occur in almost any
molluscan shells, though they are confined to certain groups, and
their color and lustre depend upon those of the shell interior,
adjacent to which they are formed. Thus the pink conch of the West
Indies yields the beautiful rose-colored pearls, while those of the
common oyster and clam are dead white or dark purple, according to
their proximity to the part of the mantle which secretes the white or
the dark material of the shell. The true pearly or nacreous iridescent
interior belongs to only a few families of the mollusks, and in these
alone can pearls proper be formed at all, while in point of fact they
are actually obtained only from a very few genera.
According to William H. Dall,1
none of the air-breathing mollusks (the land snails) produce a nacreous
shell; and among fresh-water mollusks, none are pearl-bearers except
certain of the bivalves, notably those belonging to the groups
appropriately called the Naiades, of which the common river-mussel
(Unio) is a typical example. The soft internal parts of these mollusks
are covered by a thin, delicate membrane called the mantle, from the
surface, and particularly from the outer edges of which, material is
excreted to form the inner layers of the shell. The shell consists of
two parts, the epidermis and the shell proper, the latter composed of
numerous layers. The epidermis, which resembles horn, is chiefly
composed of a substance called "conchioline" and is soluble in caustic
alkalies.
The
families with iridescent interior layers are the following : Among
cephalopods, the Nautilus and Ammonites, the latter wholly fossil. In
both these groups the removal of the outer layers of the shell reveals
the splendid pearly surface beneath. Modern nautilus shells are often
"cleaned" with dilute acid to fit them for use as ornaments, and
frequently this is done partially, elaborate patterns being formed by
leaving parts of the white middle layers to contrast with the pearly
ground. Among the fossil Ammonites, the same effect is produced, very
often naturally by decay of the outer layers ; and no artificial
pearl-work can compare with the richness of color—
1 Pearls and Pearl Fisheries, American Naturalist, p. 17, pp. 579 and 731, June and July, 1883.
