Hardness is the ability of a material to scratch a mark on other substances or to resist being scratched by them. Scientists measure the hardness of a material by comparing it with a table of 10 well-known minerals. The minerals are arranged in order from 1 to 10. Each mineral in the table scratches the ones with lower numbers, and can be scratched by all those with higher numbers. The standard “scale of hardness” follows: (1) talc, (2) gypsum, (3) calcite, (4) fluorite, (5) apatite, (6) feldspar, (7) quartz, (8) topaz, (9) corundum, (10) diamond.
To test another substance, you match it against the minerals of the hardness scale. You can get an approximate idea of the hardness of a mineral by using your fingernail, a copper coin, a piece of window glass, or a knife blade. The hardness of these materials is as follows: fingernail, 2 to 21/2; copper coin, 21/2 to 3; window glass, 5 to 51/2; and knife blade, 51/2.
When materials must be accurately tested, as in the manufacture of tools and gears, machinists use an instrument called a sclerometer. This device registers the force required to dent or scratch the material with a diamond or Borazon, the hardest substances known.
