Crossword Genius

Cameo stone (4)

Ross

I think it could be either:

opal

onyx

(Other definitions for opal that I've seen before include "Form of hydrated silica, often used as a gemstone" , "Quartz-like gemstone" , "Gem-stone with a milky quality" , "Translucent gem" , "Milky-white gemstone with flashes of colour" .)

I've seen this clue in the King Feature Syndicate and the Newsday.
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