Chemical composition -- Fossilized wood or lignite coal -- essentially carbon, but this material provides quite a contrast to the best known and most popular form of carbon -- diamond!
Color -- Black.
Optics -- R.I. 1.66.
Durability -- Hardness 2.5-4. Despite its softness, jet is tough enough for use in jewelry, especially in pieces not subject to intensive wear.
Crystal structure -- Amorphous.
Specific Gravity -- 1.19-1.35.
Sources -- The most famous source is England, but other locales are widely scattered. Jet has been used traditionally for mourning jewelry, sometimes faceted despite its opacity.