The hardness of a mineral (or precious stone) is its resistance to being scratched by another, and to measure it is used the so-called Mohs Scale. The article by Esperanza Blanco for  www.profes.net explains us the details.

The hardness, like the rest of the physical properties of minerals, it is useful to distinguish some minerals from other with simple trials and practical way.

170 years ago, Friedrich Mohs, a French mineralogist, established a hardness scale of 1 to 10, in which 1 is the softest mineral, and 10 the hardest.

Following previous practice standards, a mineral hardness 5 is harder than the hardness minerals 1, 2, 3 and 4 and, therefore, be able to scratch them. For its part, the hardness minerals 6, 7, 8, 9 and 10 will be able to scratch it.

Thus, it is easy to establish the hardness of a mineral either: if you scratch a mineral hardness 3 and is scratched by a mineral hardness of 5, then our mineral hardness is 4.

The values ​​of hardness Mohs scale are represented by a specific mineral. Fot these minerals, Mohs assigned hardness values ​​using criteria such as those already used in past practice: if scratched with a fingernail, with the copper coin, with the glass, steel, etc...