Hack stone (4)
I believe the answer is:
jade
This clue is a double definition.
'hack' is the first definition.
(I know that hack can be written as jade)
'stone' is the second definition.
(I know that jade is a type of opaque gem)
(Other definitions for jade that I've seen before include "Semi-precious gemstone -- tired old horse" , "Wear down" , "Semiprecious stone - shade of green" , "horse not in prime condition" , "Greenish, semi-precious stone" .)