Crossword Genius

Form of chalcedony (4)

Ross

I believe the answer is:

onyx

'form of chalcedony' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for onyx that I've seen before include "Semi-precious variety of agate" , "Type of agate" , "Marbled stone with layers of colours" , "Black variety of quartz" , "Smooth stone with bands of colour" .)

I've seen this clue in The Times and The Sunday Times.
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