The TSPROF Boride CS-HD five-piece sharpening stone set contains five sharpening stones made from silicon carbide on aluminium plates. Silicon carbide is a seriously hard sharpening material that can effortlessly sharpen knives with a hardness up to 63 HRC. Even the hardest Japanese kitchen knives are no match for these sharpening stones. Compared to aluminium oxide, silicon carbide has sharper and thinner particles. This results in less wide scratches and a nice silky shine on the edge of your knife when using higher grit sizes.