... Optical glass, vitrify, gem, agate, honing cylinder sleeve and other hard friable materials.4.Please mark in ... , durableSuitable for form grinding, knives, Measuring instrumentGreen silicon carbideGCgreenOnly no harder than Boron carbideSuitable for grinding Hard ... glass, vitrify and other hard friable materials.Black silicon carbideCblack Harder than corundum, crisper, less flexibility.Suitable ...