Gem with a "matrix" variety (4)

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "Semi-transparent gemstone" , "Semi-precious stone of milky or bluish colour" , "Gem-stone with a milky quality" , "item in Hatton Garden?" , "Birthstone for October" .)

I've seen this clue in the USA Today.

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