Mohs Scale of Mineral Hardness

What is the hardest mineral in the world? How do we know how hard each mineral is?

Well in terms of the 'Mohs Scale of Mineral Hardness' (which is what I've based this post on), it is based upon the scratch resistance of numerous minerals through the ability of harder minerals to scratch softer minerals.

The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material.

On this scale, 1 is the softest, whilst 10 is the hardest.

1. Talc

 Credit to R.Weller/Cochise College

Mohs scale hardness: 1

Absolute Hardness: 1

Talc is the softest known mineral and is composed of hydrated magnesium silicate ( H₂Mg₃(SiO₃)₄ or Mg₃Si₄O₁₀(OH)₂).

When in its loose form, it is known as talcum powder. Its colour ranges from white to grey or green.

Talc is a metamorphic mineral.

Some of its uses are: paper making, plastic, paint and coatings, ceramics, food, electric cable, pharmaceuticals, cosmetics and rubber.

2. Gypsum

Mohs scale hardness: 2

Absolute Hardness: 3

Gypsum is composed of calcium sulfate dihydrate, with the chemical formula CaSO₄·2H₂O. It is a very soft mineral. If heated enough, gypsum will melt and will form plaster of Paris.

Gypsum occurs in nature as flattened and often twinned crystals and transparent cleavable masses called selenite.

Gypsum can be used for: Gypsum board, plaster ingredient, plaster of Paris, fertiliser, a binder in fast-dry tennis court clay, adding hardness to water for homebrewing, a component for Portland cement, an ingredient for making mead, foot creams, shampoos and other hair products, and impression plasters in dentistry.

3. Calcite

Mohs scale hardness: 3

Absolute Hardness: 9

Calcite is a carbonate mineral (CaCO₃). It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form.

It will dissolve with most forms of acid, it was used in World War II for gun sights, specifically in bomb sights and anti-aircraft weaponry.

4. Fluorite

Mohs scale hardness: 4

Absolute Hardness: 21

Fluorite (also known as fluorspar) is a halide mineral composed of calcium fluoride, (CaF₂). The melting point of pure calcium fluoride is 1676 K. Fluorite may occur as a vein deposit, especially with metallic minerals, where it often forms a part of the gangue.

Notable fluorite deposits occur in China, Germany, Austria, Switzerland, England, Norway, Mexico, and both the Province of Ontario and Newfoundland and Labrador in Canada.

It can be used for ornamental and lapidary purposes, jewellery,  in some high performance telescopes instead of glass, exposure tools for the semiconductor industry, and fluorescence microscopy.

5. Apatite

Mohs scale hardness: 5

Absolute Hardness: 48

Apatite is a group of phosphate minerals, named for high concentrations of OH⁻, F⁻, Cl⁻ or Br⁻ ions.

It is one of only a few minerals that is produced and used by biological micro-environmental systems. The primary use of the material is to manufacture fertiliser.

During the 'Apollo Program', moon rocks collected by astronauts were found to have traces of apatite within. 

6. Orthoclase Feldspar

 Credit to R.Weller/Cochise College

Mohs scale hardness: 6

Absolute Hardness: 72

Orthoclase ( KAlSi₃O₈) is a tectosilicate mineral which forms igneous rock. It can be used for manufacturing of some glasses, some ceramics (such as porcelain), and as a constituent of scouring powder.

7. Quartz

 Credit to R.Weller/Cochise College

Mohs scale hardness: 7

Absolute Hardness: 100

After feldspar, Quartz is the second-most-abundant mineral in the Earth's continental crust.

It is composed of silicon–oxygen tetrahedra (SiO₄). There are many diverse varieties of quartz, some which are precious gemstones. It is the most common material for making jewellery and hardstone carvings.

8. Topaz

 Credit to R.Weller/Cochise College

Mohs scale hardness: 8

Absolute Hardness: 200

Topaz is a silicate mineral of fluorine and aluminium with the chemical formula (Al₂SiO₄(F,OH)₂). Pure topaz has no colour, however, impurities are still often present. Natural pink topaz is rare and naturally occurring blue topaz is also quite rare.

Some clear topaz crystals from Brazilian pegmatites can reach boulder size and weigh hundreds of pounds. Topaz is a popular gemstone.

9. Corundum

 Credit to R.Weller/Cochise College

Mohs scale hardness: 9

Absolute Hardness: 400

Corundum is a rock-forming mineral and crystalline form of aluminium oxide (Al₂O₃) with traces of iron, chromium and titanium.

It is a naturally transparent occurring material, but when impurities are present, can be different in colouration.

Because of corundum's hardness, it can scratch almost every other mineral. It is used as an abrasive, on everything from sandpaper to large machines used in machining metals, plastics, and wood.

10. Diamond

 Credit to R.Weller/Cochise College 

Mohs scale hardness: 10

Absolute Hardness: 1,600

Diamond is an allotrope of carbon, and is the hardest mineral on the planet. It also has the highest thermal conductivity temperature. 

Because of its extremely rigid lattice, it can be contaminated by very few types of impurities, such as nitrogen and boron.

Out of all the gemstones, because of its efficient marketing, and brilliant optical and mechanical properties, diamond is the most popular. 

Most natural diamonds are formed in the Earth's mantle. Diamonds are brought close to the Earth's surface through deep volcanic eruptions.

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