How a material's hardness is determined by measuring the size of an indentation made by a sharp diamond pressed strongly onto a material specimen.
For context, the hardness of alumina ceramics is nearly three times that of stainless steel; silicon carbide is more than four times harder than stainless steel. This extreme hardness is one of many unique properties that makes Kyocera's Fine Ceramics "super materials" for modern technology.
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https://global.kyocera.com/fcworld/charact/strong/hardness.html