Image credit: Scott Brande
The Bottom Line on Hardness - Here on Top!
Scratching a mineral sample against a glass plate is a quick-and-dirty test for relative hardness. If the mineral scratches the plate, it is harder; if it doesn't, it is softer. Of course the hardness test could be ambiguous under some minerals that show a hardness about the same as a glass plate. A number of common, non-metallic minerals exhibit a relative hardness less than a glass plate - H < 5.5, such as calcite and gypsum. Some metallic minerals are also softer than a glass plate, such as graphite and native copper.
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What's Mohs Hardness?
A short video on Mr. Mohs and his hardness test.
Background - The hardness of a mineral is its resistance to scratching or indenting. A simplified and crude test for hardness is to test whether or not a sharp corner or edge of a sample scratches (or indents) a glass plate. A numerical reference scale for hardness was devised by Mohs. A glass plate has a hardness of about 5 1/2 on the Mohs hardness scale.
Watch the video on the Mohs Hardness scale.
Video Demonstration for Relative Hardness
How do you DO a hardness test?
Watch the video demonstration below.
Minerals aren't the only solids that may be characterized by a relative value on the hardness scale. Other solids could be subjected to a scratch test, such as on a glass plate, for their hardness response.
Some common materials are located on the hardness scale (below). Note their assigned values. Recognize that a manufactured item may vary in its physical properties. As a consequence the value shown should be interpreted with a plus/minus to represent some variability dependent on the precise composition of the manufactured item.
The common objects are referred to as 'tools' by geologists. A field geologist normally carries a pocket knife for a quick hardness test of rocks and minerals.
Note the hardness values of these 'tools'.
steel blade of a pocket knife
masonry drill bit
A Mohs Hardness Scale
As shown on the illustration below, 10 values on the hardness scale are represented by reference minerals. Also shown are the approximate values of some common tools.
Instructions: The Hardness Test
Caution - The accurate determination of relative hardness by testing the mineral against a glass plate requires a sample with a relatively sharp edge or corner, a firm grip, a significant downward force, and a steady hand. For example, if one does not exert enough downward force (a "wimpy" scratch), one might conclude that the sample is softer than a glass plate, when in fact it is not. Perhaps a weekly visit to the local gym for some upper body strength training would be in order to help with this test!
Materials for test
Procedure for the test
Hold the sample in one hand to expose a sharp corner or edge.
Steady the glass plate on the table with your other hand.
Press the mineral sharp corner or edge onto the glass with some force.
Drag the sample across the plate a short distance (don't "saw" the sample back-and-forth like cutting a piece of wood).
Examine the glass surface for evidence of an indentation. Drag a fingernail or other hard object at right angles across the scratch line. If the sample really did scratch the glass, one will feel an indentation into the glass.
Possible test results and interpretation
If an indented groove (scratch) is present, then the mineral exhibits a Mohs hardness greater than (>) 5.5.
If an indented groove (scratch) is absent, then the mineral exhibits a Mohs hardness less than (<) 5.5.