The four organic gemstone groups listed below are highly prized for
their beauty and rarity. However, they are not as durable as gemstones
from minerals:
Amber (hardness: 2-2.5 Mohs)
A mixture of hydrocarbons
Specific gravity: 1.05-1.096
Hard fossil resin or sap of ancient pine trees. Usually amorphous (lacks
crystalline structure). Sometimes mined, sometimes gathered on
seashores.
Varies from transparent to semitransparent and generally from light
yellow to dark brown, but can be orange, red, whitish, greenish-brown,
blue, or violet. Can be dyed in any color.
Takes a fine polish. Used mainly in making beads or other ornaments.
Coral (hardness: 3.5-4 Mohs)
Formed mainly of calcite (calcium carbonate) or conchiolin, a horny
organic substance
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
Specific gravity: 2.71
Pearl-bearing mollusks are found in both salt and fresh water.
Salt-water pearls of gem quality are usually preferred for jewelry; they
are produced almost entirely by the mollusk _Pinctada_. Fresh-water
pearls are produced by various clams and mussels.
Natural pearls come in various shapes: round, pear, drop, egg, and
others. They also come in various colors, such as white, cream, light
rose, cream rose, black, gray, bronze, blue, dark blue, blue green, red,
purple, yellow, and violet.