Gemstone that can come from petrified wood (4)

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "Form of hydrated silica, often used as a gemstone" , "Type of gem" , "Milky white gem" , "Milk-coloured gemstone" , "Milky iridescent stone" .)

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

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