Crossword Genius

Stone with silica (4)

Ross

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "Variegated gemstone" , "Gem stone" , "Shimmering stone" , "Opaque mineral used a gemstone" , "what may be set in 29 [DIAL]" .)

I've seen this clue in The New York Times.
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