Gem whose name comes from "upala," the Sanskrit word for "precious stone" (4)

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "item in Hatton Garden?" , "Semi-transparent gem" , "Gem-stone with a milky quality" , "Shimmering stone" , "Variegated gemstone" .)

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

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