Nods, X-ray turning out a gem (8)
I believe the answer is:
sardonyx
'a gem' is the definition.
(I know that sardonyx is a type of agate)
'nods x-ray turning out' is the wordplay.
'x ray' becomes 'x-ray' (synonyms).
'turning out' is an anagram indicator.
'nods'+'xray'='nodsxray'
'nodsxray' with letters rearranged gives 'SARDONYX'.
(Other definitions for sardonyx that I've seen before include "Mineral" , "Semi-precious stone" , "A stone; dry Saxon (anag.)" .)