Ex-PM: ‘Time to go, having gained ten stone (4)
I believe the answer is:
onyx
'stone' is the definition.
(type of gemstone)
I can't explain the remainder of the clue.
Can you help me to learn more?
(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" .)