Crossword Genius

Turn out mineral to produce gemstone (10)

Ross

I believe the answer is:

tourmaline

'gemstone' is the definition.
(tourmaline is a kind of gemstone)

'turn out mineral to produce' is the wordplay.
'turn' indicates an anagram.
'to produce' is an anagram indicator.
'out' anagrammed gives 'tou'.
'mineral' anagrammed gives 'rmaline'.
'tou'+'rmaline'='TOURMALINE'

(Other definitions for tourmaline that I've seen before include "Beautiful blackish mineral" , "Beautiful grey or black mineral" .)

I've seen this clue in The Telegraph.
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