The solid shell of a mollusc is composed of thin layers of calcium carbonate, in the form of aragonite or calcite. Lamellated aragonite structure is paraller to the internal surface and this nacreous inner layer forms the familiar mother-of-pearl-lining.Not all the shells furnish the mother-of-pearl but some of them have, however, nacreous inner lining. Because of this particularity they are used by different kinds of craftsmen for various purposes. To reveal the mother-of-pearl gleam below, it is necessary to rub down or take of using a corrosive acid the surface (ostracum, non-iriscend) of the shell. Sometimes the next layer (periostracum, a sort of a chitine cuticle) needs also to be taken of. Both bivalves and gastropods can be pearl-bearing molluscs. Pearls can be obtained both from the marine and fresh-water species, which often furnish a high grade mother-of-pearls, though sometimes calcite is mixed into the process with the result of irregular iriscence. Therefore the pearl may not be  so spectacular. Some shell’s iridescent layer is very thick while the others have only a thin nacreous lining.The nacreous shells are able to furnish a pearl from the same materials as they produce their own shells. The best pearls are found in the large Pacific Pearl Oyster, Pinctata margaritifera. After that the best pearls can be obtained from Trocus niloticus  (used by the button makers), Turbo marmoratus  and Haliotis. Among the countless uses of mother-of-pearl may be mentioned inlay work of all kinds, ranging from articles of furniture to snuff boxes, functional items suhc as buttons, combs and knife handles and luxury items such as spectacle cases, scent bottles, playing-card cases and fans.