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Moh's scale of hardness

Short answer

Hardness is a mineral’s resistance to scratching. Mohs scale arranges ten minerals from talc (1) to diamond (10), each scratching those below it. Common field indicators: fingernail (~2.5), copper coin (~3.5), knife/glass (~5.5), steel file (~6.5).

Define hardness of minerals. Describe Moh's scale of hardness.

Hardness of a mineral is its resistance to scratching (abrasion). It depends on the strength of the chemical bonds and the atomic structure of the mineral.

Mohs scale

In 1812 Friedrich Mohs proposed a relative scale of ten standard minerals, in which each mineral scratches those below it.

HardnessMineralSimple test
1TalcScratched by fingernail, soapy feel
2GypsumScratched by fingernail
3CalciteScratched by copper coin
4FluoriteScratched by knife easily
5ApatiteScratched by knife with difficulty
6Orthoclase feldsparScratches glass with difficulty; scratched by steel file
7QuartzScratches glass easily
8TopazScratches quartz
9CorundumScratches topaz
10DiamondHardest; scratches all

Field aids

  • Fingernail: about 2.5
  • Copper coin: about 3.5
  • Knife blade / glass: about 5.5
  • Steel file: about 6.5

Notes

  • It is a relative, ordinal scale: the gap between 9 and 10 is much larger than between 1 and 2.
  • To test an unknown mineral, find which standard mineral or object scratches it and which it scratches; its hardness lies between them.
  • Test a fresh surface and not a weathered or powdery one.

✓ Answer checked by Claude Opus 5.5 (high effort)

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