Semi-precious variety of agate (4)
I believe the answer is:
onyx
'semi-precious variety of agate' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for onyx that I've seen before include "Agate with different bands of colour" , "Stone-like marble with layered colours" , "Quartz used as a gem" , "Variety of agate, used as a gemstone" , "Stone-like marble with coloured layers" .)