Black gemstone (4)
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" .)