"Interference," as it is called, is an optical phenomenon arising from two causes. When light falls upon a sufficiently thin transparent surface covering a denser substratum not exactly parallel with it, part of the light is at once reflected. Of that which passes through to the under surface a part also is in turn reflected through the first surface, and the confusion of rays or "interference" resulting, produces to the eye the sensation of color.
VENEZUELAN PEARL-SHELL, WITH PEARL ATTACHED
A familiar illustration is seen when a thin film of oil is spread over water. The other way in which iridescence by interference is produced in shells, may be demonstrated by drawing fine lines close together on glass with a diamond. Light falling upon them will make the surface iridescent. Melted wax dropped upon this striated surface would, upon removal, show a like iridescence, reproduced with the 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 coarser in construction and quality as they approach the outer surface.
In the inside of the shell, the calcium carbonate plates are very fine and transparent, and the animal membrane in which they are set is of extreme tenuity. In the middle shell these plates become more chalky and less compact; in the exterior shell they are set in a thicker binding of organic matter and terminate outside in rough, horny fringes, completely covering the shell.
In a general way therefore, the animal deposits the best of its secretions about itself and pushes out to the outer extremities, the coarser elements which are fitted to preserve the finer parts of the shell, as the finer parts of the shell are fitted to protect the delicate organism which they enclose. The building of the shell is done by a membraneous covering of the fish which entirely envelops the body and is attached to the shell a short distance from the inner edge, leaving a rim of membrane free around the fish and the edges of the two valves. This membrane is called the mantle. It extracts lime from the water, and at different parts exudes modified solutions of it mixed with animal tissue, suitable for the construction of the various parts of the shell.
The exterior of the shell or epidermis consists of conchiolin, an organic compound. It is a horny-looking substance, and in the large salt-water shells and in most of the fresh-water mussels, the nigger-head of the Mississippi Valley especially, it appears to the eye as a series of extensions, sometimes terminating in ridges, which curve about the umbo and spread to the edge of the shell, each extension coming from under the one preceding. In some varieties it is attached as an excrescence to the prismatic formation immediately under it, and may be easily detached in thin flakes: a rusty black in some, brownish-yellow in all on the inner surface and in some on the outside. The substance is generally opaque, but contains spots of which some are translucent, resembling horn or amber, while others are more transparent, similar in formation to the inner parts of the shell.
In most of the marine and fresh-water varieties, unlike the nigger-head, the conchiolin exterior does not easily flake off. In these the outer shell is composed of wave-like plate extensions, superimposed one upon the other recedingly from the lip to the umbo as in the others, but without the ridges, the plates being flat and the edges more irregular. These extensions are formed of a number of horizontal composite plates, which penetrate the shell to the mother-of-pearl.