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 Mechanical properties |
One of the first qualities of a stone is its hardness. Any stone is classified according to the Mohs scale of hardness which gives to minerals a varying index from 1 to 10. Ten minerals are classified by order of scratch resistance (each one scratch the previous and is scratched by the following) :
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
Talc |
Gypsum |
Calcite |
Fluorine |
Apatite |
Orthose |
Quartz |
Topaz |
Corundum |
Diamond |
With part of index 7, all the minerals scratch glass. On the other hand, all those whose index is lower or equal to 2 can be scratch by a nail !
The scale of Knoop shows the force to be applied in ten minerals of Mohs to dig a notch with a diamond point there. This diagram shows us as well as the progression is nonlinear in hardness:
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| It makes it possible to know the density of a mineral. This calculation takes place by comparing the weight of a mineral with that of an equal volume of water. Thus, for an identical size, a diamond (PS=3.52) weighs heavier than an emerald (PS=2.71). |
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| There are two manners of breaking stones: cleavage and the fracture. If the crack is done while following a surface without relationship to the atomic structure, we speak about fracture, if not, we speak about cleavage. |
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