Banded chalcedony (4)
'banded chalcedony' 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 "Agate with alternating bands of colour" , "Form of chalcedony" , "Sort of agate" , "Gemstone, a variety of chalcedony" , "Stone-like marble with layered colours" .)
'banded chalcedony' is the definition.
'agate' can be an answer for 'chalcedony' (agate is a kind of chalcedony). I also know that 'banded' relates to this answer.
This is the entire clue.
(Other definitions for agate that I've seen before include "Quartz with bands of colour, used in jewellery" , "Variegated chalcedony" , "Quartz gem" , "Onyx" , "Semi-transparent stone" .)