Gemstones with a matrix variety (5)

I believe the answer is:

opals

(Other definitions for opals that I've seen before include "Milky-white stones" , "Translucent gemstones" , "Milky gemstones with delicate colours" , "Semi-precious stones of milky or bluish colour" , "Precious stones" .)

I've seen this clue in the Universal.

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