We added the normal quantity of 20 grit and ran the tumbler for a month. At the end of the month we opened the barrel. A great deal of that 20 grit had survived! Until eventually the creation of boron carbide in 1929, silicon carbide was the hardest synthetic material https://productsincludeboatholder70369.wikigdia.com/6477748/getting_my_black_silicon_carbide_in_tvs_to_work