Scratch hardness refers to the hardness of a material in terms of resistance to scratches and abrasion by a harder material forcefully drawn over its surface.
Resistance to abrasion is less affected by surface variations than indentation methods.
[1] Attempting to scratch a surface to test a material is a very old technique.
[6][8] Charles E. Wooddell, working at the Carborundum Company, extended the scale further by using resistance to abrasion, and extrapolating the scale based on 7 for quartz and 9 for corundum, resulting in a value of 42.4 for South American brown diamond bort.
[9][10] There is a linear relationship between cohesive energy density (lattice energy per volume) and Wooddell wear resistance, occurring between corundum (H=9) and diamond (H=42.5).