Hard greyish-black mineral used as an abrasive and polishing agent (5)
I believe the answer is:
emery
(Other definitions for emery that I've seen before include "Dick - - , late comedian" , "Coating on paper used as an abrasive" , "Hard black-grey mineral used in grinding and polishing" , "Abrasive form of corundum" , "Black abrasive" .)