Exploit quality of rocks but not diamonds? (7)
I believe the answer is:
harness
'exploit' is the definition.
(I know that harness is a more specific form of the action exploit)
'quality of rocks but not diamonds' is the wordplay.
'quality of rocks' becomes 'hardness' (rocks are hard).
'but not' is a deletion indicator.
'diamonds' becomes 'd' (abbreviation in bridge say).
'hardness' with 'd' taken out is 'HARNESS'.
(Other definitions for harness that I've seen before include "Set of straps for horse etc" , "Control; trappings" , "Horse's tackle" , "riding equipment" , "Equipment for a draught animal" .)