Jack ruined spare gemstone (6)
I believe the answer is:
jasper
'gemstone' is the definition.
(I know that jasper is a type of opaque gem)
'jack ruined spare' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'ruined' indicates an anagram.
'spare' is an anagram of 'asper'.
'j'+'asper'='JASPER'
(Other definitions for jasper that I've seen before include "Gemstone" , "A man" , "stone used by jewellers" , "Precious stone - boy's name" , "hard matter" .)