Gemstone - quite sour if processed (9)
I believe the answer is:
turquoise
'gemstone' is the definition.
(I know that turquoise is a type of opaque gem)
'quite sour if processed' is the wordplay.
'if processed' indicates anagramming the letters.
'quite'+'sour'='quitesour'
'quitesour' is an anagram of 'TURQUOISE'.
(Other definitions for turquoise that I've seen before include "not quite green" , "Aquamarine colour" , "Birthstone for December" , "Greenish-blue colour" , "Greenish blue; gemstone" .)