Different methods are the base for hardness tests of minerals. Commonly used and internationally known is the Mohs (mineralogist, 1773-1839) hardness scale. It depends on the discovery that softer substances scratch harder ones. So Mohs created his scale, arranging 10 minerals according to their scratching hardness from the softest material (talcum) with hardness 1 up to the hardest material (diamond) with hardness 10. The Mohs scale is a relative scale. Later other scales were developed which show the absolute hardness: The Rosiwal scale e.g. shows the expenditure during the cutting ("cutting hardness"), the Vickers (airplane factory) scale shows the denting depth of a material.
Regarding gemstones the Mohs hardness is always meant when dealers and buyers meet.



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