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time. Struggling with its burden, it literally shingles the pearl with
minute, almost translucent white patches. The edges of these
multitudinous applications, reflecting rays of light, produce the effect
which we all iridescence. (2)

The value of a pearl depends on more than iridescence. In
appraising a pearl, aside from size or weight, jewelers consider a perfect
skin and a fine "orient." In other words, it must possess a delicate
texture and be without a speck or flaw. The color must be clear and of
white translucence with a subtle, iridescent sheen. It must also be
uniformly spherical or at least uniformly pearshaped. The form or shape
of natural pearls, because of the accidental nature of their origin, is often
far from perfection. Often the pearl is attached to the inside of the shell.
In this position only the exposed part will receive the nacreous deposit
and the result is a pearl which is flat on one side and rounded on the
other, known in the trade as a "perle bouton." Sometimes when a
mollusk is attacked by a boring animal, it deposits matter over the point
of attack. The result is a hollow, nacreous body of irregular shape called
a "blister pearl." A hollow, warty pearl is often called "cog de perle,"
while a solid pearl, not symmetrical in form, such as might form around a
bit of sea-weed or wood, is called a "perle baroque," or a barrok pearl.
(1)

The most precious pearls and the best mother-of-pearl are
obtained from the oyster of the genus Margaritiferus, species and