The scale of hardness used was devised by Mohs. It assigns numbers to ten minerals--the lowest being softest, the highest being hardest. The numbers and substances are:

1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum
10. Diamond

Hardness values for the elements were obtained from the 67th edition of the HANDBOOK OF CHEMISTRY AND PHYSICS, R. C. Weast, Ed., CRC Press, Inc., 1986, page F-19.