Mineral gemstone (5)
I believe the answer is:
beryl
'mineral gemstone' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for beryl that I've seen before include "Mineral, aquamarine if blue" , "Green stone" , "Pale greenish" , "Transparent mineral used as a gemstone" , "Gemstone of which emerald is a variety" .)