Purple quartz gemstone (8)
I believe the answer is:
amethyst
'purple quartz gemstone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for amethyst that I've seen before include "Violet-coloured stone" , "what's in ring" , "Bluish-violet 3" , "Precious stone of purple quartz" , "Purple quartz used in jewellery" .)