Type of chalcedony used in jewellery (4)

I believe the answer is:

onyx

(Other definitions for onyx that I've seen before include "Variety of agate used for making cameos" , "Form of quartz" , "Marble-like gemstone" , "Mineral with coloured layers" , "Hard stone with coloured layers" .)

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

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