What is Mohs hardness scale?

In 1812, mineralogist Friedrich Mohs developed a scale for determining the hardness of minerals. He ranked the 10 most common minerals in the descending order according to their ability to scratch each other. Mohs assigned the lowest value on the scale to the softest mineral, talc, which can be scratched by all the other minerals higher on the scale. The highest on the scale is diamond, which is rated ten and can scratch all the minerals below it. The Mohs scale is not proportional and is only used to quickly assess the relative hardness of natural stone.

The atoms that make up the structure of minerals come in different sizes and valences. These two factors tell a lot about the physical properties of the particular stone. The hardness of the stone can be determined by scratching, drilling, pressing and grinding its surface. It may vary in the stones of the same type, as this property depends on the proportions of minerals minerals that make them up and overall impurities present in the stone. The Austrian mineralogist Mohs (1772-1839) listed 10 minerals in relation to each other in terms of hardness as this: 

1. Talc (the softest)
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond (the hardest)

The scale is relative because the hardness of diamond is 150 times that of corundum and 1 million times that of talc. According to famous Lithuanian geologist Vincentas Korkutis, the approximate hardness of a mineral can be also measured as this: if the surface can be scratched with a pencil, it equals to 1 on the Mohs scale; if it is scratched with a fingernail, it is 2; with a copper coin, it is 3 to 4; with a glass comb, it is 5; with a steel knife, it is 6; with a file, it is 7 to 8. Sometimes the hardness of the same mineral can vary by two points when measured in different directions.

The Mohs scale shows that diamond is the hardest mineral. It is very resistant chemically - nor acids nor alkali solutions make it etch.