Earth Science
The Mohs Hardness Test

   During the early 1800s, a German mineralogist named Friedrich Mohs devised a scale that tested mineral hardness, which means the resistance of a mineral to being scratched. This scale, which ran from 1 to 10, was named after Mohs, and is known as the Mohs Hardness Test. (Hardness should not be confused with brittleness. Diamond, the hardest known natural substance, can produce a scratch on virtually all other substances, but can easily be shattered by the blow of a hammer.) Keep in mind that the Mohs scale does not indicate exact hardness, which depends upon the purity of the mineral sample and its degree of crystallinity. The Mohs Scale is shown below, using commonly known minerals to represent each degree of hardness: While a reference set of the above minerals may be used to test the hardness of an unknown mineral, such a set is rarely used in actual practice; particularly because diamond is rather expensive to be used as an abrasive. Instead, common objects are used as tools to perform the hardness test: Using the above testing tools, you should be able to determine the Mohs Hardness of an unknown mineral to the nearest half number on the scale. Keep these principles in mind when performing the hardness test: How to perform the scratch test: You should find a sharp corner of a mineral and scratch the smooth surface of the test object (fingernail, penny, glass plate, knife, steel file, etc.) This is the preferred method, and this will preserve the quality of the mineral specimens. If this is not possible, you can also try to scratch the smooth surface of the unknown mineral using a sharp point of the testing tool. When performing the hardness test, you should always start from the soft end of the scale - that is, try using your fingernail first rather than the steel file. Why? The steel file is able to scratch minerals with hardness of 6.5 down to 1, and does not give you any precise information. A fingernail can scratch only a few minerals. When the test fails, the unknown mineral is then harder than your fingernail. You should logically ask, "How much harder than your fingernail?" and proceed to the next harder object, the penny. Suppose the unknown mineral easily scratches the penny. Which is harder, the mineral or the penny? The mineral is obviously harder than 3.0 - how much harder? Proceeding with the glass plate, the mineral is unable to scratch the glass (hardness = 5.5) at all. This mineral is about 4.0 on the Mohs scale. (If a mineral indeed scratches glass, you will be unable to wipe the scratch away.)

Copyright ©2000 by William K. Tong