Ex-PM: ‘Time to go, having gained ten stone (4)

I believe the answer is:

onyx

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'stone' is the definition.
(type of gemstone)

I can't explain the remainder of the clue.

(Other definitions for onyx that I've seen before include "Quartz with alternating bands of colour" , "Chalcedony agate" , "Marble-like gemstone" , "Mineral with coloured layers" , "Stone-like marble with layered colours" .)

I've seen this clue in The Guardian.

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