Mohs Hardness Scale - Testing Procedure

Additional Information

The hardness test developed by Friedrich Mohs in 1812 was the first known test to assess resistance of a material to scratching. It is a very simple but inexact comparative test. Perhaps its simplicity has enabled it to become the most widely used hardness test. The test compares the resistance of a mineral to being scratched by ten reference minerals that range from a very soft mineral (talc) to a very hard mineral (diamond). With the exception of diamond, the minerals are all relatively common.

A set of hardness picks can be used to perform this test. The picks have sharp metal points that you can use for very accurate testing. The picks allow much control, and their sharp points can be used to test small mineral grains in a rock. An alternative to the hardness picks (which can be costly) are these common objects (fingernail, copper, nail, glass, knife blade, steel file, streak plate, or quartz). Before using these objects as testing tools, it is a good idea to confirm their hardness.

The common objects listed above have the hardness as follows:

Fingernail - 2 to 2.5

Copper - 3

Nail - 4

Glass - 5.5

Knife blade - 5 to 6.5

Steel file - 6.5

Streak plate - 6.5 to 7

Quartz - 7

Mohs simply selected ten minerals that varied in hardness and arbitrarily placed them on an integer scale from 1 to 10, they are as below:

Talc - 1

Gypsum - 2

Calcite - 3

Fluorite - 4

Apatite - 5

Orthoclase - 6

Quartz - 7

Topaz - 8

Corundum - 9

Diamond - 10

The test is conducted by placing a sharp point of one specimen on an unmarked surface of another specimen and attempting to produce a scratch.

Locate a smooth, unscratched surface for testing.
With one hand, hold the specimen of unknown hardness firmly against a table top so that the unscratched surface to be tested is exposed and accessible.
Hold one of the known hardness testing objects in the other hand and place a point of that specimen against the selected flat surface of the unknown specimen.
Firmly press the point of the known hardness object against the unknown specimen, and firmly drag the point of the standard specimen across the surface of the unknown specimen.
Examine the surface of the unknown specimen. With a finger, brush away any mineral fragments or powder that was produced. Did the test produce a scratch? Be careful not to confuse mineral powder or residue with a scratch. A scratch will be a distinct groove cut in the mineral surface, not a mark on the surface that wipes away.
Perform the test again to confirm your results.
Practice and experience will improve your ability doing this test, you will become faster and more confident.

helpful tips

Hardness is the resistance to scratching. During testing, some materials might break, deform, or crumble instead of scratching. A lack of toughness in hard materials could result in breakage when subjected to stress. Other materials might deform or crumble when subjected to stress, these materials lack strength. Keep this in mind and remember that you are testing for the resistance to being scratched.

A reference list of minerals in order of hardness can be a handy. If you determine that a specimen has a hardness of Mohs 4, you can quickly get a list of possible minerals.

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