Material hardness is commonly understood as resistance to scratching or abrasion. However, different tests are used to measure different aspects of material hardness in accordance with the mechanical properties under investigation. Moreover, tests are conducted under varying experimental conditions and data analysis methods. Consequently, no direct relationship exists between the results of different hardness tests. The most common is “Mohs Test” which measures “scratch hardness” on a comparative scale of 10 reference minerals. The principle is simple: material A will scratch material B only if A is harder than B. Common objects of known hardness can be used to perform Mohs test.
Press firmly into, and across the surface of the glass with a fingernail. Not surprisingly, you find that it cannot be scratched by a fingernail. This means on Mohs scale, glass is harder than 2.5.
Continue the test using the penny for scratching. Note that the penny fails to scratch the glass. You then conclude that glass has a Mohs hardness greater than 3.
Refer to the tips section for a list of Mohs test reference minerals in order of hardness, followed by the hardness of the common objects you will use next.
Test the materials against themselves. Note that a given material will scratch only materials of lower hardness.
Continue the tests using the nail for scratching, then the apatite, and so on, in increasing order of hardness until you have fixed the glass between two reference materials.
Note that neither the nail nor the apatite scratches the glass, but orthoclase does. You can then conclude that glass has a Mohs hardness between 5 and 6.