Jasper or opal (3)
I believe the answer is:
gem
'jasper or opal' is the definition.
(I know that jasper is a type of opaque gem)
This is the entire clue.
(Other definitions for gem that I've seen before include "Diamond, ruby, or emerald" , "Half of 7 [SIGN OF THE ZODIAC]" , "perfect specimen" , "Treasured person" , "Emerald or sapphire, say" .)