Geode stone (5)
I believe the answer is:
agate
'geode stone' 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 "Banded ornamental stone" , "Semi-transparent stone" , "Semi-precious quartz" , "Striped stone" , "Impure form of quartz with banded chalcedony, used as a gemstone" .)