Banded mineral (5)
I believe the answer is:
agate
'banded mineral' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for agate that I've seen before include "Variety of chalcedony" , "Semi-precious quartz" , "Banded ornamental stone" , "Quartz variety" , "Variegated chalcedony" .)