Using Hardness to Identify Minerals

Determining the hardness of an unknown rock or mineral is often very useful in the identification process. Hardness is a measure of a mineral's resistance to abrasion and is measured against a standard scale - Mohs Scale of Hardness. Mohs Scale was named after Frederick Mohs, a German minerologist. It consists of 10 fairly common minerals of known hardness which are numerically ordered from the softest (1) to the hardest (10). They are:

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

As common sense dictates, Mohs Scale is based on the fact that a harder material will scratch a softer one. By using a simple scratch test, you can determine the relative hardness of an unknown mineral.

Your fingernail has a hardenss of 2.5. If you can scratch the surface of an unknown specimen with it, you will immediately know that its hardness is less than 2.5. In other words, it is slightly harder than gypsum (H=2) but softer than calcite (H=3).

A penny has a hardness of 3.0 - slightly harder than your fingernail. So, if you can't scratch the specimen with your fingernail (H=2.5), but a penny does the job, you immediately know that it is at least as hard as calcite (H=3).

The steel blade of the average knife usually has a hardness of about 5.5. If a penny does not scratch your unknown specimen but the knife blade does, then you can correctly conclude that it is harder than calcite (H=3) but softer than orthoclase (H=6).

Your wife’s diamond ring (H=10) may come in handy at this point and it would be nice of you to invite her along with you. Unless, of course, you are one of those cheap &*%$ who bought her a cubic zirconium and don’t want to have the truth out just yet.