238 GEMS AND PRECIOUS STONES IN THE
little
south of San Diego, containing over a hundred tons of shells, from
exposure to the rain and the sun was converted into lime on the
outside, but this was broken into and many fine shells were found.
The
shell in its natural state is no more attractive than that of the
oyster; it is rough on the outside, looking much like a piece of dried
brown clay, and is frequently covered with a growth of barnacles,
seaweeds, etc. Commercially there are five varieties, the green, the
black, the red, the pink, and the mottled; but considering them from an
ornamental standpoint, the shells may be grouped under three heads,
red, black, and green, so-called, of course, from their prevalent
color. The black, which is the smallest and least valuable, is found
from Monterey down to the Gulf of California; the red, which is next in
value, but the largest in size, is found from Mendocino to Monterey ;
while the green comes from below San Diego. The black seldom exceeds 6
inches in diameter, the green rarely goes beyond 9, while the red runs
as high as 12 or 14 inches. The black is not beautiful on the outside,
even when cleansed of lime and marine parasites, but inside there lies a. small
patch of the most beautiful opalescent tints, and this is sawn out, and
made into brooches and lockets. The red is of a general mother-of-pearl
appearance, with stripes and mottles of a rich burnt umber. The green,
both within and without, is full of fire and color, some interiors
being fully as vivid and of much the same prevailing color as a
peacock's neck. This variety is principally used for jewelry, and is
worked into every kind of ornament, from a table-top, inlaid with
representations of flowers and butterflies, to the smaller varieties
of jewelry. The Pueblo, Zuni, and Navajo Indians, and all the Indians
of the California coast as far north as Alaska, have made it into
charms and have used it for ornamentation for ages. It has been used as
an applied decoration on silver objects, and was exhibited at the
World's Fair held in Paris during 1889.
The
play of colors is attributed by Sir David Brewster to minute striae or
grooves on the surface of the nacre alternating with the grooves of
animal membrane. These laminae decompose the light in consequence of
the interference caused by the
