The most widely used grinding disks are made of silicon carbide (chemical formula SiC, hardness nº 9 in the Mohs scale, black color). A very well known brand name of silicon carbide is Carborundum, who were actually the first producers of that material.
Now, silicon carbide has a problem when used for stainless steel: the metal (alloy, rather) that has been in contact with the disk becomes contaminated with carbon, and thus prone to intergranular corrosion.
So, for ginding stainless steel, a more suitable material should be used, and this is commonly alumina (chemical name aluminum oxyde, formula Al2O3, hardeness nº 9 in Mohs scale, white color). Of course, the metal (alloy, rather) becomes contaminated with aluminum, but this is not harmful, or if it is, it is certainly much less than contamination with carbon.
Giovanni S. Crisi