Hardness is a characteristic property of the solid objects. It is measured by the resistance which the body offers to anything which tends to scratch it. The hardness of the various materials is measured either on the ‘Mohs’ scale or the ‘Knoop’ scale.
The Mohs’ scale, first devised in 1822 by Friedrich Mohs, measures resistance to indentation as judged by the material that will scratch another. Mohs’ scale is numbered from 1 to 10, that is, it gives ten grades of hardness. In this scale diamond is the hardest material and talc is the softest. Diamond has a hardness of 10 Mohs and talc has a hardness of 1 Mohs.
Mohs’ scale, which assigns numbers to natural minerals, has been widely accepted and is used by mineralogists. This test, however, is not quantitative. For example, the hardness of sapphire is 9 on the Mohs’ scale; it does not mean that sapphire is 10% softer than diamond.
The mineralogists carry a box containing pieces of the above minerals for testing samples in the field. For example, if they find a mineral that can be scratched by feldspar but not by appetite, its hardness lies between 5 and 6 on the hardness scale.
To measure hardness in the Knoop scale, an elongated diamond-shaped indenting device is employed to measure the indentation it makes in a given test material. By this method, the hardness of extremely brittle materials including glass and even diamond can be measured without damaging either the indenter or the test piece. The size of the indentation is taken as a measure of the material’s hardness.