Banded semiprecious stones (6)
I believe the answer is:
agates
'banded semiprecious stones' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for agates that I've seen before include "Ste aga up for the stones" , "Hard coloured stones from a stage" , "Multi-coloured quartzes" , "[RING]-fillers required" , "Pieces of impure quartz used as gemstones" .)