The hardness of the stones

The hardness of the stones

From vector properties of minerals, especially gemstones, the most important thing is hardness. It is resistance, that minerals put when trying to scratch or wear them. Very soft minerals are known, that can be scratched with a fingernail or a match stick; medium hard minerals, which can be scratched easily with the blade of a pocket knife, finally, hard minerals, that cannot be scratched with a penknife, and sometimes even very hard, scratching glass. Hardness is an important property, which makes it possible to recognize and distinguish minerals with a similar appearance.

In order to easily and quickly determine the hardness, a conventional hardness scale consisting of 10 minerals, called the Mohs scale. Minerals that differ significantly in hardness are arranged in this way, that the harder the mineral, the further it ranks in the hardness scale: 1 — talk, 2 - cast, 3 — kalcyt, 4 — fluoryt, 5 - apatyt, 6 - orthoclase, 7 - quartz, 8 — topaz, 9 — korund, 10 — diament.

Minerals with the same hardness scratch each other (although it is not easy). Therefore, if the examined mineral scratches orthoclase, which on the Mohs scale is a reference mineral with hardness 6, and at the same time it may itself be scratched by the orthoclase, it means, that both minerals have the same hardness, that is, the examined mineral has a hardness 6. If the tested mineral has a hardness intermediate between two consecutive minerals on the hardness scale, it is defined by an integer that relates to the softer mineral on the scale, with the addition of a fraction of half a degree, e.g.. 7,5 or 7½.

Please note, that the Mohs hardness scale is only a relative qualitative scale, i.e.. that mineral, whose hardness is indicated by a number 2, it is not at all 2 times harder than a mineral, whose Mohs hardness is given by a number 1. Likewise, orthoclase is not 6 times harder than talc or 2 times harder than calcite. In fact, the differences in the hardness of individual minerals on the hardness scale vary, in some positions a lot (sometimes several dozen or several hundred) times higher than the hardness of the preceding mineral. For the precise quantitative measurement of hardness, instruments called hardness testers are used for special purposes only. Scratching the mineral with a steel or diamond stylus occurs depending on the load, the size of which is a measure of hardness.

Not having a hardness scale set of minerals, the following approximate indicative scale can be used:

Degree on the Mohs scale Mineral

standard

Orientation study
1. Talk it can be scratched with a piece of wood, e.g.. match
2. Cast it can be scratched with a fingernail
3. Kalcyt it can be easily scratched with a copper coin
4. Fluoryt it can be easily scratched with the blade of a pocket knife
5. Apatyt it can be scratched with the blade of a pocket knife
6. Feldspar it can hardly be scratched with the blade of a pocket knife
7. Quartz draws glass (window glass)
8. Topaz easily scratches quartz
9. Korund it can be scratched with a diamond, topaz doesn't scratch it
10. Diament it cannot be scratched by any mineral or artificial substance

From the exact quantitative hardness measurements it follows, that the property is a vector property, i.e.. depending on the direction. However, the differences in hardness of the same material are usually so slight, that they can be omitted. However, they can sometimes have some meaning, e.g.. when grinding large diamonds. Because in some directions the hardness of diamonds is greater, and less in others, grinding them with diamond powder can scratch the smooth surface of the stone. That's why big, very expensive stones must be ground very carefully, after determining the direction, in which grinding will not damage the smooth surface.

Most gemstones are harder than quartz. Sometimes, however, for decorative purposes, minerals with a hardness lower than quartz are used, characterized by a beautiful color and a strong shine. Stones of lower hardness are called often semi-precious stones.

Formerly, when optical research has not yet been used to identify gemstones, are currently the most important methods of testing these stones, a hardness test was often used for this purpose, using special pencils, the ends of which were reference minerals in the Mohs hardness scale. This method is now rarely used both because of inaccuracy, and because of the possibility of scratching and damaging the tested stones, especially large and polished. However, it may sometimes render certain services, e.g.. during field work, when it is impossible to determine the density or to test the optical properties of the minerals.

The hardness of gemstones according to the Mohs scale.