Crossword Genius

Nods, X-ray turning out a gem (8)

Ross

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.)" .)

I've seen this clue in The Guardian.
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