Layered gemstone with flashes of colour in light (4)

I believe the answer is:

opal

(Other definitions for opal that I've seen before include "raw material" , "what may be set in 29 [DIAL]" , "Type of gem" , "Mineral used as a gemstone, of hydrated silica" , "Milky-white gemstone with flashes of colour" .)

I've seen this clue in The Irish Times.

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