Aquamarine, for one (5)
I believe the answer is:
beryl
'aquamarine for one' is the definition.
(I know that aquamarine is a type of beryl)
This is the entire clue.
(Other definitions for beryl that I've seen before include "- Bainbridge, English novelist" , "Transparent mineral used as a gemstone" , "Girl's name - precious stone" , "Pale greenish" , "Girl's name - gemstone" .)