impression
of the fine lines. The outer markings of the large Australian shell are
similar to the small Venezuelan. The mother-of-pearl interior is not so
iridescent.
Pearls
and the shells in which they grow are composed almost entirely of
calcium carbonate or lime. A small percentage of organic matter and
water are the other ingredients.
As
pearls are accidental and the result of a misdirection of normal
processes, a general knowledge of those processes is necessary to an
insight into the nature and genesis of the pearl, and as pearl shells
and the pearls in them are constructed on the same general plan, a
knowledge of the former will assist to a better understanding of the
gem and its eccentricities. The mother-of-pearl shell is built up of a
series of calcium carbonate plates or prisms set in organic matter. In
the material of the inner shell, the calcium carbonate greatly
preponderates; on the outside of the shell, the organic matter is
largely in excess. In the building of its shell, the animal deposits
the finest material and does the best and most compact work where the
house is in touch with itself, the walls becoming
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