Crossword Genius

Black gemstone (4)

Ross

I believe the answer is:

onyx

'black gemstone' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for onyx that I've seen before include "Type of agate" , "Marbled stone with layers of colours" , "Marble-like gemstone" , "Agate with different layers of colour" , "precious stone" .)

I've seen this clue in the USA Today, the Universal and the LA Times.
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