... holdout, durableSuitable for form grinding, knives, Measuring instrumentGreen silicon carbideGCgreenOnly no harder than Boron carbideSuitable for grinding Hard alloy ... Optical glass, vitrify and other hard friable materials.Black silicon carbideCblack Harder than corundum, crisper, less flexibility.Suitable ... strength metallic and Nonmetallic material, i.e. single alundum silicon carbide fiber disc