The outer hard covering of certain molluscs, turtles, etc., which is used for various purposes, including making or decorating certain objects of jewelry. The shell of molluscs is generally referred to, when applied to objects of jewelry (e.g. brooches and finger rings), as 'shell', but that of turtles as tortoise shell. Shell is sometimes carved as a cameo.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson