As hard as it comes
The hardest and the sharpest abrasive known. This is a closeup of 45 micron, 325 mesh monocrystalline diamond stone. The Mohs scale of mineral hardness gives diamond a 10 out of 10, so well done diamonds.
I have a few of these diamon stones to sharpen chisels, a wood plane and even ski edges and they are much more effective than the oil stone my Dad had. That was silicon carbide and came in at a wimpy 9.5 out of 10.