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Hardness sounds like a simple property, yet comparing it needs a method. Friedrich Mohs gave mineralogists one that needs no machine, only reference minerals and a careful hand. His scale is over two hundred years old and still in use. This post covers who he was, what he actually wrote about the scale, a worked example of using it, and the mistake students most often make with it.
Who was Friedrich Mohs?
The Neue Deutsche Biographie, Germany's national biographical dictionary, gives his dates as 29 January 1773, in Gernrode in the Harz, to 29 September 1839, at Agordo near Belluno. He went to the University of Halle at 23, then in 1798 to the mining academy at Freiberg. In 1812 he began arranging the mineral collection of the Joanneum in Graz, where he became a professor. In 1818 he succeeded Abraham Gottlob Werner in the chair of mineralogy at Freiberg, and in 1826 he moved to the University of Vienna.
The NDB entry says that in 1812 Mohs set up a hardness scale of minerals with ten degrees, later named after him and still used internationally. It adds that he classified minerals by their outward characters, much as Linnaeus classified living things, and rejected chemistry as a criterion.
What did Mohs write about his scale?
His own description is in the English edition of his Treatise on Mineralogy (1825), translated by William Haidinger. Mohs explains the rule: choose minerals so that each one is scratched by the one that follows it, while it cannot scratch that one. Using his names, with the modern ones in brackets, the scale runs:
Talc
Gypsum (prismatoidal Gypsum-haloide)
Calcite (rhombohedral Lime-haloide)
Fluorite (octahedral Fluor-haloide)
Apatite (rhombohedral Fluor-haloide)
Feldspar (prismatic Feld-spar, as the variety adularia)
Quartz
Topaz
Corundum
Diamond
He notes that gypsum has exactly the hardness of rock salt, and that the state of being liquid can be treated as zero on the scale.
Did Mohs think the steps were equal?
No, and he said so. In the Treatise he writes that "the intervals between the members of the scale are not everywhere of the same magnitude", and that diamond is "evidently much harder" compared with corundum than fluorite is compared with calcite. He also found the gap between apatite and feldspar larger than it should be, and wished he had another mineral to put there.
How did Mohs actually test hardness?
He did more than scratch minerals against each other. In the Treatise he warns that scratching alone is not reliable enough, because corners and faces vary. His method was to try scratching the members of the scale with a corner of the unknown mineral, find the first one it clearly scratches, and then compare the two on a fine, hard file by the resistance and the sound. He estimated hardness to tenths where needed, though he said it would seldom be required to better than 0.5. He also warned that some minerals are softer on a cleavage face than in other directions.
Worked example: placing an unknown mineral
Here are sample test results for an unknown grey mineral:
It scratches fluorite (4).
It does not scratch feldspar (6), and feldspar scratches it.
It and apatite (5) scratch each other only faintly.
Step 1: bracket it. Harder than 4, softer than 6, so its hardness lies between 4 and 6. Step 2: narrow it. Mutual faint scratching with apatite means it is close to 5. Step 3: report it. Hardness about 5. Following Mohs's advice, you would only add a decimal, such as 5.5, after further comparison.
What you cannot say: "it is 25% harder than fluorite". The scale does not support that arithmetic.
What mistakes do students make with the Mohs scale?
Treating the numbers as a ratio scale. Order is all the scale gives.
Confusing hardness with toughness. Hardness is resistance to scratching. A hard mineral can still break when struck.
Testing on a weathered surface. Mohs said impure or decomposed specimens give wrong results.
Hardness matters beyond identifying minerals. NCERT's Class 11 chapter on geomorphic processes lists the hardness or softness of constituent minerals among the rock properties that control how fast weathering and erosion work.
What can a student learn from Mohs?
He built a tool that anyone could use, then wrote down its weaknesses honestly. That habit, stating the limits of your method alongside the result, is what makes a measurement trustworthy. Use it in your practical file: note what your instrument cannot detect, not just what it shows.
If you want to see how bonding and crystal structure explain why diamond is so hard, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing.
Try it now: write down how you would place a mineral that scratches quartz but not topaz.
