The hardness of a mineral indicates how well it resists scratching and abrasion. The degree of hardness is determined by observing the comparative ease or difficulty with which one mineral is scratched by another, or by a file or knife. It is measured on a numerical scale introduced by Mohs, from 1 (softest) to 10 (hardest):
If the mineral under examination is scratched by the knife-blade as easily as calcite its hardness is said to be 3; if less easily than calcite and more so than fluorite its hardness is 3.5. In the latter case the mineral in question would be scratched by fluorite but would itself scratch calcite. It need hardly be added that great accuracy is not attainable by the above methods, though, indeed, for purposes of the determination of minerals, exactness is quite unnecessary.
It should be noted that minerals of:
- grade 1 have a soft, greasy feel to the hand like talc and graphite, and flakes of it will be left on the fingers,
- grade 2 are easily scratched by the fingernail, as a cleavage piece of gypsum,
- grade 3 are rather readily cut, as by a knife and just scratched by a copper coin but is not scratched by the fingernail,
- grade 4, scratched rather easily by the knife but is not so easily cut as calcite,
- grade 5, scratched with some difficulty,
- grade 6, barely scratched by a knife, but distinctly by a file - moreover, they also scratch ordinary glass,
- minerals as hard as quartz (H. = 7), or harder, scratch glass readily but are little touched by a file.
- Chesterman, Charles W. The Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf: New York (1987)
- Dana, Edward S. and Ford, William E. Dana's Textbook of Mineralogy - Fourth Edition, John Wiley and Sons: New York (1932)
- Hurlburt, Jr., Cornelius S. Minerals and How to Study Them - Third Edition, John Wiley and Sons: New York (1958)