Mohs scale

An ordinal scale ranking minerals by resistance to scratching, from talc at 1 to diamond at 10. Its values indicate relative order, not equal intervals of hardness.

Scratch test: A test that assesses a material’s resistance to being scratched by another material. The scale’s rankings come from determining which mineral scratches another.
Talc: A very soft mineral composed of hydrated magnesium silicate, assigned hardness 1 on the Mohs scale. It anchors the scale at its softest reference point.
Friedrich Mohs: An Austrian mineralogist who introduced a mineral hardness scale in 1812. He devised the scale and selected its reference minerals.
Mineral identification key: A structured guide that uses observable properties to distinguish and identify minerals. Hardness is one diagnostic property used in mineral identification keys.
Absolute hardness: A quantitative measure of resistance to indentation or scratching, rather than a rank in an ordinal sequence. It clarifies why equal differences between Mohs numbers do not mean equal hardness differences.
Mohs hardness test: A comparative test that uses reference materials to determine a mineral’s position on the Mohs scale. This procedure applies the scale to an unknown mineral by testing scratch relationships.
Gypsum: A soft sulfate mineral composed of calcium sulfate dihydrate, assigned hardness 2 on the Mohs scale. It is the next standard mineral above talc.
1812: A year in the early nineteenth century, notable in mineralogy for Friedrich Mohs’s introduction of his hardness scale. The scale’s historical introduction is commonly dated to this year.
Field geology: The study of geological materials and structures through observations made outdoors. Scratch comparisons provide a portable way to assess minerals in the field.
Knoop hardness test: A microhardness test that measures the elongated indentation made by a diamond-shaped indenter. It offers a quantitative method suited to small or brittle samples.

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