Rocks & Minerals

The relative degree of ‘scratchability’ or susceptibility to scratching can be assessed using the Mohs' scale of hardness. In 1822 Friedrich Mohs, a German mineralogist, took ten minerals that he was able to obtain easily and tested them against each other. Mohs arranged the minerals in order, from the softest (talc) that could be scratched by all the others, to the hardest which could not be scratched (diamond). He then assigned them numbers from the softest (1 = talc) to the hardest (10 = DIAMOND).

The test is a comparative one, and the scale is not linear and does not increase by equal increments, for example the difference in hardness between 1 (talc) and 9 (the corundum group, which includes ruby and sapphire) is less than that between 9 and 10 (diamond).

Other hardness scales include: a) the Brinell Scale, which measures how much of a dent can be made when a steel ball is pushed into the surface (this cannot be used on particularly brittle, thin or fragile pieces, as they would break); b) the Knoop scale which uses a diamond to make a measurable dent; and c) the Vickers scale (see graph) which also uses a diamond to make a dent.

The Mohs' test for hardness is quicker, cheaper and easier than the other methods. An estimate of hardness can be given by looking at the general wear and tear of the crystal faces (on an uncut ‘rough’ specimen) or flat surfaces (facets) and facet edges on a cut and polished gemstone. A set of hardness pencils (each with a point made of one of the ten mineral specimens) can be used to test hardness and is particularly useful on carved pieces, crystal fragments and pebbles.

From CREDO Mohs' Scale of Hardness in Guide to Gems