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The power of a gemstone, mineral, glass, or other hard object to resist being abraded, i.e. the cohesion of the particles on the surface of such a body, as determined by its capacity to scratch another or to resist being scratched.
Most gemstones, which possess hardness to a high degree, have an almost uniform degree of hardness; but such hardness may vary slightly in different directions or for the same species of stone from different localities. The hardness is measured by comparison with that of other selected minerals according to the Mohs' scale; it is indicated by the symbol 'H' followed by the numeral indicating its rating on the scale. Hardness can be tested by a jeweller by use of hardness plates or hardness points.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson