The Mohs scale of mineral hardness categorises gems by the various levels of scratch resistance.  The scale runs from one to ten, with diamond being the hardest mineral with a rating of ten. 

Mohs Scale of Hardness:

1.  Talc

2.  Gypsum

3.  Calcite

4.  Fluorite

5.  Apatite

6.  Orthoclase

7.  Quartz 

8.  Topaz

9.  Corundum (Ruby and Sapphire)

10. Diamond