Hardness is measured on the Moh's hardness scale. Each of the values is a naturally occuring mineral. Testing devices (e.g. pencils) mount these in the tip.
Ceramic glazes are the obvious candidate for this type of testing. If a specimen to be tested can be scratched by a mineral from the list, it is softer than that mineral. If it in turn will scratch another known mineral, it is harder than that mineral.
Background Information on Mohs Hardness
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