Jet

Meaning: Jet is a black organic substance which is a fossilized variety of brown coal, formed as a result of the compaction of driftwood over millions of years.
Jet has a dull lustre in rough form while vitreous lustre after polished. It is a stone with low S.G. (1.20 - 1.30) but high R.I. (1.66). Theses are the key identifying features of jet for separating it from its simulants.

Properties

Chemical Composition: Carbon with impurities but not a mineral.

Classification / Type: A compact form of coal (further heat and pressure makes it more tough)

Colors / Varieties: Black, brownish.

Crystal System / Forms: None. Amorphous.

Hardness: 2.5 - 4

Specific Gravity:1.20 - 1.30

Cleavage / Fracture: None / Conchoidal. 

Optic Character: Isotropic (S.R.)

Lustre: Dull in rough; vitreous when polished.

Refractive Index / Birefringence: 1.66 (approx.) / Nil.

Pleochroism: None.

Dispersion: None. 

Magnification: None.

U.V. Fluorescence: Inert

Spectrum: None.

Treatment (Enhancement): None.

Specific Tests & Remarks: 

• Gives a strong coal or oily odor to a hot point.
• Brown streak color on a porcelain plate.

Synthesis: None.

Simulants (with key separation tests): 

• Anthracite (black streak)
• Glass (R.I., S.G., inclusions)
• Plastic (R.I., hot point, structure)
• Black stained chalcedony (R.I., S.G., banding)
• Obsidian (R.I., S.G., inclusions)
• Vulcanite - hard rubber (rotten egg smell to a hot point)

Geological Occurrence: In sedimentary deposits, it is a fossilized wood.

Sources: England, Spain, France, U.S.A., Germany, Russia.

Cuts & Uses: Carvings, beads, cabochons, etc.