Turning out mineral for gemstone (10)
I believe the answer is:
tourmaline
'gemstone' is the definition.
(tourmaline is a kind of gemstone)
'turning out mineral' is the wordplay.
'turning' is an anagram indicator.
'out'+'mineral'='outmineral'
'outmineral' with letters rearranged gives 'TOURMALINE'.
'for' is the link.
(Other definitions for tourmaline that I've seen before include "Beautiful grey or black mineral" , "Beautiful blackish mineral" .)