Banded rock (5)
I believe the answer is:
agate
'banded rock' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for agate that I've seen before include "Quartz gem" , "On the way" , "Quartz with bands of colour, used in jewellery" , "Semi-transparent stone" , "marble" .)