Shell Jewelry
The outer hard covering of certain mollusks, turtles, etc., is used for making shell jewelry. The shell of mollusks is generally referred to simply as “shell”, but that of turtles as tortoise.

From the outside, the shell may look plain and unattractive, but when opened, it reveals a shimmering, iridescent interior made of nacre, the substance produced by certain mollusks such as abalone. Nacre is also what mollusks use to form pearls.

A little advice from JeGem: using a damp soft cloth to clean your shell jewelry can ensure it brings you a life-time of pleasure. Never use abrasives or chemicals or an ultrasonic cleaner which will damage the shell.