Type of chalcedony (4)
I believe the answer is:
onyx
'type of chalcedony' 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 "Stones" , "Quartz with alternating bands of colour" , "Form of agate" , "precious stone" , "Black variety of quartz" .)