Jet (gemstone)
Mineraloid and minor gemstone / From Wikipedia, the free encyclopedia
Jet is a type of lignite,[1] the lowest rank of coal, and is a gemstone. Unlike many gemstones, jet is not a mineral, but is rather a mineraloid.[2] It is derived from wood that has changed under extreme pressure.
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Jet | |
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Sample of unworked jet, about 15 mm long | |
General | |
Category | Mineraloid |
Formula (repeating unit) | Variable, but rich in carbon |
Identification | |
Color | Black, occasionally brown |
Cleavage | None |
Fracture | Conchoidal |
Tenacity | Brittle |
Mohs scale hardness | 2.5–4.0 |
Streak | Brown |
Specific gravity | 1.3–1.4 |
Optical properties | Isotropic |
Refractive index | 1.640–1.680 |
Dispersion | None; opaque |
Ultraviolet fluorescence | None |
Common impurities | Iron, sulfur |
References | [citation needed] |
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The English noun jet derives from the French word for the same material, jaiet (modern French jais), ultimately referring to the ancient town of Gagae.[3] Jet is either black or dark brown, but may contain pyrite inclusions[4] which are of brassy colour and metallic lustre. The adjective "jet-black", meaning as dark a black as possible, derives from this material.