Form of agate (4)
I believe the answer is:
onyx
'form of agate' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for onyx that I've seen before include "Agate with alternating bands of colour" , "Marble-like gemstone" , "Semi-precious stone used in making cameos" , "Variety of agate, used as a gemstone" , "Banded chalcedony" .)