An organic material (not a shell) that is obtained, not from a tortoise, but from the overlapping horny top plates (called 'blades') covering the carapace (upper shell) of certain marine turtles, preferably the hawksbill turtle found of the West Indies and Brazil and the loggerhead turtle found near the Celebes.
Such plates are translucent and of a dark-brownish colour marbled with yellow. A yellow (blond) variety is less often preferred. Tortoise shell can be moulded by heating (but excessive heat darkens the colour) and also thickened or enlarged by joining heated pieces under pressure. It has been used for inlaying, e.g. piqué and coulé (glued) decoration, and for making, since Roman days, various personal articles (e.g. combs) and pieces of jewelry (e.g. brooches, bracelets, earrings, bangles), including pieces carved as a cameo.
Imitation tortoise shell is made from plastic and stained horn, but is identifiable by the smell from burnt samples or by microscopic examination which reveals in imitation an absence of small, spherical, reddish particles.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson