Iron amulet needs resetting for gemstone (10)
I believe the answer is:
tourmaline
'for gemstone' is the definition.
(tourmaline is a kind of gemstone)
'iron amulet needs resetting' is the wordplay.
'needs resetting' is an anagram indicator.
'iron'+'amulet'='ironamulet'
'ironamulet' is an anagram of 'TOURMALINE'.
(Other definitions for tourmaline that I've seen before include "Beautiful blackish mineral" , "Beautiful grey or black mineral" .)