Gemstone quiets our rioting (9)
I believe the answer is:
turquoise
'gemstone' is the definition.
(I know that turquoise is a type of opaque gem)
'quiets our rioting' is the wordplay.
'rioting' indicates anagramming the letters.
'quiets'+'our'='quietsour'
'quietsour' is an anagram of 'TURQUOISE'.
(Other definitions for turquoise that I've seen before include "not quite green" , "Precious stone" , "Greenish blue; gemstone" , "Blue-green" , "Greenish-blue stone" .)