Designer's Guide Results
The following is a list of materials, selected and ranked per your specification.

MaterialHardness
Boron Carbide2800-3000 Knoop
Titanium DiBoride2700-2800 Knoop
Silicon Carbide Sintered2550-2800 Knoop
CVD Silicon Carbide2540 Knoop
Silicon Carbide Reaction Bonded2500-2550 Knoop
Sapphire2200 Knoop
perpendicular to the c-axis
1900 Knoop
parallel to the c-Axis
Alumina 99.9%2000 Knoop
Alumina 99.5%2000 Knoop
Alumina 94%2000 Knoop
Zirconia Toughened Alumina (ZTA)1750-2100 Knoop
Reference Materials
Diamond9000 Knoop
Tool Steel650-900 Knoop
Stainless Steel92 Rockwell B

All properties values are at room temperature unless otherwise noted. Engineering data are representative, and are not intended as absolute nor warrantable. Data shown is blended from multiple sources and therefore illustrates the marketplace. The relative rankings are approximate and are based on representative values if the particular material has a range of values for a given property.

Wikipedia Definition(s) :  Hardness;

Check Different Properties

Please Note:  This Designers guide is compiled of materials currently machined by Insaco and a few others included for reference.