Hardness on the Mohs scale is determined by the ability of one mineral to scratch another. Hardness is not a measure of a mineral’s strength, weight, toughness, or ease of breaking. The softest mineral used as a reference standard is talc. Diamond is the hardest reference mineral. Mohs hardness numbers indicate an order of comparison, not uniform intervals. That is, the hardness difference between 1 and 2 is not the same as the difference between 9 and 10.
A scratch must be left for the hardness comparison to be meaningful. Powder and metallic marks are not scratches. A powdery mark will rub off easily. When making the test, apply only gentle pressure, make a short test line, and clean the test area before using a hand lens to look for the scratch. Applying more pressure is unlikely to give a more accurate test and may cause the specimen or the testing tool to be damaged.
Use a small and common specimen with a dull surface. Do not use delicate crystals, coated specimens, valuable specimens, or any other specimens where you wish to avoid marking the surface. Wherever possible, choose a spot to test that will be less noticeable if marked. In multigrain or matrix specimens, be sure you do not test the wrong mineral.
When you wish to do a more precise comparison, select two reference materials of known approximate hardness which will not damage your specimen. Try the lower hardness material first. If it doesn’t scratch, try the higher. For example, suppose the specimen is not scratched by your fingernail but is scratched by the copper material. The hardness is somewhere between the hardness of those two materials. Don’t try to determine a specific Mohs hardness based on this information. Just record a range.
In addition to whether or not the mineral will scratch the reference material, also check whether the reference material will scratch the mineral. A rough edge on a hard mineral can appear to be able to scratch a soft material, but sometimes it’s just the loose crystals causing the scratch. Wipe off both surfaces and try again. Doing a second test on another section of the specimen might confirm the results of the first test and eliminate any doubt as to whether the previous test was successful due to dirt, weathering or a non-flat surface.
Don’t let hardness become the primary basis for identifying the mineral. Include properties such as luster, streak, transparency, cleavage, fracture and crystal habit in your test plan. Stop testing when you’ve got enough data to know what range of hardness a mineral fits in. More scratches don’t give you more information; they only leave more damage on your specimen.
