Mineral Hardness: Mohs Hardness Scale
History of the Mohs Scale
Mohs' scale of mineral hardness quantifies the scratch resistance of minerals by comparing the ability of a harder material to scratch a softer material. The Mohs scale was invented in 1812, by the German mineralogist Friedrich Mohs. Mohs based his scale on ten minerals.
With the Mohs Scale, the hardness of a material is measured against the scale by finding the hardest material that it can scratch, and/or by identifying the softest material that can scratch it. If a given material can be scratched by topaz, but not by quartz, its hardness on Mohs scale is 7.5.
Mohs Hardness vs True Hardness
The table above shows a comparison with absolute hardness measures using a sclerometer. The Mohs scale is a ordinal or successive scale and therefor, does not measure or compare actual hardness. For instance, corundum is twice as hard as topaz, but diamond is almost four times as hard as corundum yet there is only one step between each of these three minerals.
Minerals in Order of Hardness
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1. Talc
Talc is the world's softest mineral and the lowest mineral on the Mohs scale. Talc is a hydrated magnesium sheet silicate which is highly insoluble in water. Talc is translucent to opaque with a iridescent or pearly luster. Talc is used in cosmetics such as talcum powder, as a lubricant, and in paper manufacturing.
Absolute Hardness: 1
Chemical Composition: Mg3Si4O10(OH)2
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2. Gypsum
Gypsum is a soft mineral composed of calcium sulfate dihydrate. Gypsum occurs in nature as flattened or twinned crystals and transparent cleavable masses called selenite. When Gypsum has a silky and fibrous texture it is called Satin Spar.
Absolute Hardness: 2
Chemical Composition: CaSO4á2H2O
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3. Calcite
Calcite is an anhydrous carbonate, and one of the most widely distributed minerals on the Earth's surface. It is a common constituent of sedimentary rocks. In crystallized form, Calcite has a vitreous luster.
Absolute Hardness: 9
Chemical Composition: CaCO3
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4. Fluorite
Fluorite (fluor-spar) is a mineral composed of calcium fluoride. It is an isometric mineral with a cubic crystal habit. Fluorite is named for its property of fluorescence, or its ability to fluoresce under ultraviolet light.
Absolute Hardness: 21
Chemical Composition: CaF2
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5. Apatite
Apatite (hydroxylapatite, fluorapatite, chlorapatite) is a group of phosphate minerals and is one of few minerals that are produced by biological organisms. Hydroxylapatite is the major component of tooth enamel.
Absolute Hardness: 48
Chemical Composition: Ca5(PO4)3(OH-,Cl-,F-)
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6. Orthoclase
Orthoclase (aka feldspar) in an igneous rock forming tectosilicate (silicate) mineral and is a key component in granite. Orthoclase derives its name form the Greek word for "straight fracture" because of its two cleavages at right angles to each other. Orthoclase crystallizes in the monoclinic crystal system.
Absolute Hardness: 72
Chemical Composition: KAlSi3O8
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7. Quartz
Quartz is one of the most common minerals found in the Earth's crust. It has a hexagonal crystal structure made of trigonal crystallized silica (silicon dioxide). The typical shape of a Quartz crystal is a six-sided prism that ends in six-sided pyramids.
Absolute Hardness: 100
Chemical Composition: SiO2
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8. Topaz
Topaz is a silicate or "nesosilicate" mineral created from a combination of aluminium and fluorine. It crystallizes in the orthorhombic system and it's crystals are prismatic in form.
Absolute Hardness: 200
Chemical Composition: Al2SiO4(OH-,F-)2
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9. Corundum
Corundum is the crystalline form of aluminium oxide and one of the basic rock-forming minerals. Corundum is naturally clear or colored by impurities. Due to its hardness, Corundum is used as an abrasive in sandpaper. Emery is an impure and less abrasive variety of Corundum.
Absolute Hardness: 400
Chemical Composition: Al2O3
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10. Diamond
Diamond is the hardest natural occurring material. Diamond is a natural allotrope of carbon. The crystal bond structure of diamonds give the stone its hardness and differentiates it from graphite, which is the main allotrope of carbon.
Absolute Hardness: 1500
Chemical Composition: C
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