Amorphous mineral used as a gemstone (4)

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "what may be set in 29 [DIAL]" , "Mineral, usually iridescent" , "Shimmering stone" , "Semi-transparent gemstone" , "Iridescent gemstone" .)

I've seen this clue in The Guardian.

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