Crossword Genius

Stone with "fire" and "water" varieties

Ross

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "Semiprecious stone of white and bluish colour" , "One's stony" , "little jewel" , "Milky-white mineral" , "Precious stuff" .)

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