Milky gemstone (4)

I believe the answer is:

opal

'milky gemstone' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for opal that I've seen before include "Delicately coloured gem" , "Iridescent quartz-like stone" , "Semi-transparent gem" , "Milky-white gemstone with flashes of colour" , "Type of gem" .)

I've seen this clue in the Penny Press, The Wall Street Journal, The New York Times and The Sun.

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