Black jet usually has a shine, while coal is usually more of a
solid black.
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Jet is a stone similar to coal, but harder, and is basically
black. One presumes then the origin of "jet black".
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you can take a small sample of fuel and light a match and put it
to the fuel. coal fuel will ignite whereas jet fuel will extinguish
the flam of the match.
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Jet is a form of coal (lignite); onyx is a variety of silicon dioxide.
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Jet is a gem quality variety of lignite coal and would be
brownish to black.