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A scale for measuring the hardness (resistance to abrasion or scratching) of a specimen of mineral or other hard substance, prepared in 1812 by the Austrian mineralogist Friedrich Mohs (1773-1839), being a series of ten specified minerals in the order of their relative hardness - the intervals between the listed minerals not being equal or proportionate - against which the specimen is rated.
The minerals on the scale are: talc; gypsum, calcite, fluorspar, apatite, orthoclase (feldspar); quartz; topaz; sapphire (corundum); diamond. Intermediate measurements are indicated by fractions. All the important gemstones have a hardness of at least 5, but pearls and coral are lower.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson