Crossword Genius

Jet-black gem

Ross

I believe the answer is:

onyx

'jet-black gem' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for onyx that I've seen before include "Form of agate" , "Mineral with coloured layers" , "Black variety of quartz" , "Agate with different bands of colour" , "Variety of agate, used as a gemstone" .)

I've seen this clue in The New York Times.
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