Emerald or aquamarine (5)
I believe the answer is:
beryl
'emerald or aquamarine' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for beryl that I've seen before include "Transparent mineral used as a gemstone" , "Gemstone of which emerald is a variety" , "Blue, green or yellow mineral, sometimes a gemstone" , "Mineral, aquamarine if blue" , "Emerald or aquamarine" .)