Kind of quartz (4)
I believe the answer is:
onyx
'kind of quartz' 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 "Black variety of quartz" , "Variety of agate, used as a gemstone" , "Marble-like gemstone" , "Agate with different layers of colour" , "Black and white striped gemstone" .)