White gem (4)

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "Iridescent quartz-like stone" , "what may be set in 29 [DIAL]" , "Iridescent gemstone" , "Rock" , "precious jewel" .)

I've seen this clue in the USA Today, the King Feature Syndicate, the Newsday.com and the Newsday.

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