A crystal of calcite can be scratched easily with a knife. If we try the same with a quartz crystal, the knife slides over its surface. The two minerals have a different hardness.
To determine the hardness of a mineral, a hardness scale was established by Frederich Mohs (1773-1839). This scale has ten grades in which each higher grade of mineral can always scratch the one just below it in hardness.
The set of standard minerals used as examples for the hardness tests are:
1. talc
2. gypsum
3. calcite
4. fluorite
5. apatite
6. orthoclase feldspar
7. quartz
8. topaz
9. corundum
10. diamond
