Banded agate (4)
I believe the answer is:
onyx
'banded 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 "Form of agate" , "Form of chalcedony" , "Mineral with coloured layers" , "Stone like marble with layered colours" , "Marble-like gemstone" .)