Stripy marbles (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
agates
(Other definitions for agates that I've seen before include "Hard stones with concentric bands of colour" , "Banded semiprecious stones" , "Name for stones" , "Ste aga up for the stones" , "Stones to be found in a stage" .)