Antony cut in half unknown gemstone (4)
I believe the answer is:
onyx
'gemstone' is the definition.
(I know that onyx is a type of calcedony)
'antony cut in half unknown' is the wordplay.
'cut in half' means to take half the letters.
'unknown' becomes 'x' (algebra).
'antony' cut in half is 'ony'.
'ony'+'x'='ONYX'
(Other definitions for onyx that I've seen before include "Smooth stone with bands of colour" , "Quartz used as a gem" , "Agate with different layers of colour" , "Sort of agate" , "Stones" .)