Cameo gemstone (4)
I believe the answer is:
onyx
'cameo 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 "Stone-like marble with different colours in layers" , "Stone-like marble with coloured layers" , "Striped agate" , "Agate with alternating bands of colour" , "Form of quartz" .)