The scale of Mohs. Mohs hardness

The Mohs scale is a 10-point scale created by Carl Friedrich Mohs in 1812, which makes it possible to compare the hardness of minerals. The scale gives a qualitative, not a quantitative evaluation of the hardness of a stone.

History of creation

To create the scale, Moos used 10 reference minerals - talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, red corundum and diamond. He placed the minerals in order of increasing hardness, accepting as a starting point that the harder mineral scratches the softer. Calcite, for example, scratches gypsum, and on calcite scratches it leaves fluorite, and all these minerals cause talc to crumble. So the minerals obtained the corresponding values of hardness in the Mohs scale: chalk-1, gypsum-2, calcite-3, fluorite-4. Further studies showed that minerals whose hardness is lower than 6 are scratched by glass, those whose hardness is above 6-scratch glass . The hardness of the glass on this scale is approximately 6.5.

Stones with a hardness greater than 6 are treated with diamond.

The Mohs scale is intended only for a rough estimate of the hardness of minerals. A more accurate indicator is absolute hardness.

Location of minerals in the Mohs scale

Minerals in the scale are arranged in order of hardness. The softest has a hardness of 1, it is scratched by a fingernail, for example, talcum (chalk). Further, there are several more solid minerals - ulexite, amber, muscovite. Their hardness on the Mohs scale is small - 2. Such soft minerals are not polished, which limits their use in jewelry. Beautiful stones with low hardness refer to ornamental, and are usually inexpensive. Of these, souvenirs are often made.

Minerals with a hardness of 3 to 5 are easy to scratch with a knife. Gagat, rhodochrosite, malachite, rhodonite, turquoise, nephrite are often polished with cabochon, polished well (usually with the use of zinc oxide). These minerals are not resistant to water.

Solid jewelry minerals, diamonds, rubies, emeralds, sapphires, topazes and garnets, are processed depending on the transparency, color, presence of impurities. Starry rubies or sapphires, for example, are cut with cabochons to emphasize the unusual nature of the stone, transparent varieties are cut with ovals, circles or drops, like diamonds.

Jewelry Stones

All minerals whose hardness is less than 7 on the scale are considered soft, those above 7 are hard. Solid minerals are amenable to diamond processing, a variety of possible fences, transparency and rarity make them ideal for use in jewelry.

The hardness of the diamond on the Mohs scale is 10. Diamonds are cut in such a way that during processing the loss in the mass of the stone is minimal. The processed diamond is called a diamond. Due to its high hardness and resistance to high temperatures, diamonds are almost eternal.

The hardness of ruby and sapphire is slightly below the hardness of the diamond and is 9 on the Mohs scale. The value of these stones, as well as emeralds, depends on the color, transparency and number of defects - the more transparent the stone, the more intense the color and the less cracks in it, the higher the price.

Semiprecious stones

Somewhat below diamond and corundum, topaz and garnets are valued. Their hardness on the Mohs scale is 7-8 points. These stones are amenable to diamond processing. Price directly depends on the color. The more saturated the color of topaz or pomegranate, the more expensive the product will cost with it. The most highly valued are extremely rare yellow topaz and purple garnets (majors). The last stone is so rare that its price can be higher than a pure diamond.

Color tourmaline: pink (rubellite), blue (indigolite), green (verdelit), watermelon tourmaline is also referred to as semi-precious stones. Transparent tourmalines of high quality are very rare in nature, therefore they are sometimes much more expensive than pyrope and blue topaz, and for watermelon (pink-green) stones, they do not get tired to hunt. The hardness of the stones on the Mohs scale is quite high and is 7-7.5 points. These stones are well polished, do not change color, and finding a jewelry with a bright transparent tourmaline is a real luck.

Black variety of tourmaline (sherle) refers to ornamental stones. The shirl is a hard, but at the same time brittle stone, which can easily break down during processing. It is for this reason that black tourmalines are most often sold unprocessed. Sherl is considered the strongest protective talisman.

Industrial Applications

Minerals and rocks with high hardness are widely used in industry. For example, the hardness of granite on the Mohs scale is from 5 to 7, depending on the amount of mica in it. This solid rock is widely used in construction as a material for finishing.

Colorless sapphires or leucosapphires, despite their high hardness and relative rarity, are not in demand among jewelers, but are widely used in laser and other optical plants.

Practical application of the scale

Despite the fact that the Mohs hardness scale gives only a qualitative rather than a quantitative assessment, it is widely used in geology. Using the Mohs scale, geologists and mineralogists can approximately identify an unknown rock, depending on its susceptibility to scratching with a knife or glass. Almost all reference sources indicate the hardness of minerals on the Mohs scale, and not their absolute hardness.

In the jewelry business, the Mohs scale is also widely used. The hardness of the stone depends on the way it is processed, the possible options for grinding and the tools necessary for this.

Other hardness scales

The Mohs scale is not the only hardness scale. There are several other scales created based on the ability of minerals and other materials to resist deformation. The most famous of them is the Rockwell scale. Rockwell's method is simple - it is based on measuring the depth of penetration of the identifier deep into the material under study. As an identifier, a diamond tip is usually used. It is worth noting that the minerals are rarely subjected to the Rockwell method, usually it is used for metals and alloys.

Shor's hardness scales are constructed in a similar way. The Shore method allows to determine the hardness of both metals and more elastic materials (rubber, plastic).