Chalcedony (5)
I believe the answer is:
agate
'chalcedony' is the definition.
(agate is a kind of chalcedony)
This is the entire clue.
(Other definitions for agate that I've seen before include "Semi-transparent stone" , "Quartz with bands of colour, used in jewellery" , "Banded ornamental stone" , "Hard stone with concentric bands of colour" , "Impure form of quartz with banded chalcedony, used as a gemstone" .)