Birthstone that has a "fire" variety (4)

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "Milky-white gemstone with flashes of colour" , "raw material" , "Many-coloured gem" , "what may be set in 29 [DIAL]" , "Something milky-white" .)

I've seen this clue in the USA Today.

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