Hardness

in a description of solids, particularly minerals, the capacity of a substance to scratch another, or to be scratched by another. The diamond is the hardest body known. Hardness is frequently expressed by numerals from one to ten. Talc is at one end of the mineral scale with a hardness of 1, the diamond at the other end with a hardness of 10. According to this scale, bismuth and alum have a hardness of 2; aluminum, antimony, arsenic, bronze, copper, German silver, marble, and zinc have a hardness of 3; wrought iron, fluor spar, 4; the mineral called apatite, 5; feldspar and cast iron, 6; agate and quartz, 7; topaz and emerald, 8; tempered steel and corundum, 9; the diamond alone, 10; ice, 1 1/2. See Annealing