Specific gravity: 2.60-2.70
Each coral polyp, a tiny marine animal that lives in enormous colonies, extracts calcium carbonate from the sea and exudes it to build a protective home around and above itself. Each generation of polyps dies in its protective home and each succeeding generation builds on top of its predecessor.
Gem coral ranges from semitranslucent to opaque and occurs in white, pink, orange, red, blue, violet, golden, and black. The black and golden corals are largely horny organic substances, not calcium carbonate.
The finest coral is used to make figurines, cameos, carvings, and beads.
Jet (hardness: 2.5-4 Mohs)
Carbon plus various hydrocarbon compounds
Specific gravity: 1.30-1.32
This compact velvet-black coal takes a good polish and is often cut into beads, bracelets, and a wide range of decorative and useful objects.
Pearl (hardness: 2.5-4.5 Mohs)
Formed within a mollusk, such as an oyster, that deposits a substance called nacre around an irritant that entered the organism