50,000 to 60,000 grit is the finest diamond i use for polish. i have a 6″ batt TIN lap and i run the speed VERY slow with a VERY slow water drip. a make a muddy slur of the powdered diamond on lap with appling powder on lap with finger. we do most all stones that way and polish quickly. had no problem on spinel. and with slow speed and water we dont throw of the diamond and dry out the lap. on very large stones we sometimes use a drop or so of a light oil or crystal lube. on really large stones we’ve made hard wood laps(maple or oak) and use crystal lube and diamond on that–NO WATER–