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Hardness
In the case of minerals and gemstones, hardness refers first to scratch hardness, then to cutting-resistance. Before the development of optical testing methods, scratch hardness was of great importance in the determination of gemstones. Today the hardness test is only rarely applied to precious stones and then mainly by collectors. It is too imprecise for an expert and the danger of damaging the gem is too great. The advantage of the scratch test is that one can, with simple means, roughly determine the nature of a stone. It is frequently used in mineralogy.

The Viennese mineralogist Friedrich Mohs (1773-1839) invented the scratch hardness test. He denned scratch hardness as the resistance of a mineral when scratched with a pointed testing object. Mohs chose ten minerals of different hardness for comparison and graded these minerals one to ten. Each mineral in this series scratches the previous one, and can be scratched by the following one. Minerals of equal hardness cannot scratch each other. By comparative application of Mohs' hardness scale the hardness (according to Mohs) of every gem can be determined. Stones with scratch hardness 1 and 2 are soft, 3 to 6 medium hard, over 6 hard. Minerals of Mohs'hardness 8 to 10 are also described as "hard gems". This designation is unfortunate, as gems are characterized not only by their hardness, even if that property is valued. The luster and polish of gems of hardness below 7 can be damaged by dust as this may contain small particles of quartz (Mohs' hardness 7). Such stones must be carefully handled when worn or stored so that they do not come into contact with any scratching objects.

When a scratch test is made, care must be taken to employ a sharp-edged test piece on a fresh unbroken mineral face. Imperfect formation, foliated crystals or weathered surfaces can show a lower hardness.
Some gems show different hardness on different faces in different directions. In the case of kyanite (p. 196), Mohs'hardness along the long direction of the crystal is 4J-, while across it is 6 or 7. It is therefore also called disthene (double resistance). There are also varying hardnesses in diamond. This makes it possible to work this hardest material (see p. 59). It is important for the gem cutter and lapidary to be familiar with the various hardnesses (scratch as well as cutting resistance) as this is the basis of successful work.

Mohs' scratch hardness is a relative scale. It only shows which mineral scratches another mineral. Nothing is said about increase of hardness within the scale. These differ substantially as can be seen from the following table of absolute hardnesses (cutting hardness in water according to A. Rosiwal).

Iri the trade one can buy Mohs' hardness testers as a set in a little box. Specially recommended are hardness test points which are splinters of the mineral set in handy metal holders. The hardness value of the mineral is marked in the metal of the holder to avoid mistakes.