Diamond is renowned for its hardness. Hardness is the measure of a substance's resistance to being scratched, and only a diamond can scratch another diamond. Diamond is the hardest substance known.

The Mohs scale--a hardness scale developed in 1822 by Austrian Friedreich Mohs as a criterion for mineral identification--can help us appreciate the hardness of diamond. The scale ranks 10 minerals; harder minerals, with a higher number, can scratch those with a lower number.

When the mineral hardness numbers from the Mohs scale are plotted against those on the more quantitative Knoop scale (based on the force needed to make indentations using a diamond), we can see how it doesn't adequately express the extreme hardness of diamond. The Mohs scale is relatively stable until it reaches the eighth mineral topaz, but it jumps exponentially from corundum (colorless sapphire) to diamond. It is in fact difficult to measure the hardness of diamond, because diamond must be used to measure its own hardness.