Semi-transparent gemstone (4)

I believe the answer is:

opal

'semi-transparent gemstone' is the definition.
(I've seen this before)

This is all the clue.

(Other definitions for opal that I've seen before include "Delicately-coloured gem" , "Milky-white gemstone with flashes of colour" , "Milky white gem" , "Precious stuff" , "Milky-white gem" .)

I've seen this clue in The Guardian.

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