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Hardness
Diamond is famous for its hardness. Hardness is the measure of a substance's resistance from being scratched, and only a diamond can scrape another diamond. Diamond is the hardest material recognized.
The Mohs scale--a hardness scale developed in 1822 by Austrian Friedreich Mohs as a principle for mineral identification--can help us appreciate the hardness of diamond. The scale ranks 10 minerals; harder minerals, with a higher number, can scrape those with a lower number.
When the mineral hardness numbers from the Mohs scale are plotted touching 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 sufficiently express the extreme hardness of diamond. The Mohs scale is comparatively stable until it reaches the eighth mineral topaz, but it jumps exponentially from corundum (colorless sapphire) to diamond. It is in reality difficult to measure the hardness of diamond, because diamond must be used to measure its own hardness.
