Opaque gem (4)

I believe the answer is:

opal

'opaque gem' is the definition.
(opal is a kind of opaque gem)

This is the entire clue.

(Other definitions for opal that I've seen before include "Semi-transparent gemstone" , "Milky white gem" , "Iridescent gemstone" , "Type of gemstone" , "Milky white gemstone" .)

I've seen this clue in The Mirror.

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