Variety of agate (4)
I believe the answer is:
onyx
'variety of agate' 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 "Stone-like marble with layered colours" , "Semi-precious stone used in making cameos" , "precious stone" , "Type of agate" , "Mineral with coloured layers" .)