Colourful type of quartz (4)

I believe the answer is:

onyx

(Other definitions for onyx that I've seen before include "Cameo material" , "Banded chalcedony" , "Variety of agate, used as a gemstone" , "Stone-like marble with coloured layers" , "Stone like marble with layered colours" .)

I've seen this clue in the Sydney Morning Herald.

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