Purple gemstone (8)
I believe the answer is:
amethyst
'purple gemstone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for amethyst that I've seen before include "what's in ring" , "Violet-coloured stone" , "Variety of quartz" , "Purple quartz, used as a gemstone" , "A Middle East language" .)