

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:

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). 

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. 


