The hardness of any mineral is defined by its Mohs scale number: harder the mineral, higher its Mohs number. The Mohs scale was devised by a German geologist and mineralogist Friedrich Mohs in 1812. It works by analyzing the ability of a material to scratch the other (softer) material.
Although the Mohs scale is not precise and strictly ordinal, it has a relevant use in geology, mostly to identify various minerals. Electronic manufacturers often use this scale to test the resilience of flat panel display modules. Modern phone displays, for example, use Gorilla Glass that scratches at level-6 on the Mohs scale of hardness.
To perform the scratch test, metallurgists use sclerometer or Turner–sclerometer. Below, you will find well-studied, hardest minerals in the world, ranked according to the Mohs hardness. We have also mentioned the absolute hardness (which is measured by a sclerometer) of each mineral.
Mohs hardness: 1
Absolute hardness: 1
Chemical formula: Mg3Si4O10(OH)2
Talc is a mineral composed of hydrated magnesium silicate. It is opaque to translucent, with colors ranging from green to whitish gray. It is a common metamorphic mineral in metamorphic belts of the western United States, Western Alps, and in the Himalayan region.
Talc can be crushed into a white power, which is popularly known as ‘talcum powder.’ This powder has excellent properties for absorbing odor, moisture, and oils. The mineral also serves as a thickening agent and lubricant: it is used as an ingredient in paint, ceramics, and roofing materials.
Cesium and rubidium (with 0.2-0.3 hardness) lithium, sodium, and potassium (with 0.5-0.6 hardness) are all softer than Talc.
Gypsum found in Poland
Mohs hardness: 2
Absolute hardness: 3
Chemical formula: CaSO4·2H2O
Gypsum is a sulfate mineral composed of calcium sulfate dihydrate. It is widely mined and used to manufacture cement, plaster of Paris, wallboard, sidewalk chalk. It is also used as a fertilizer and a hardening retarder in portland cement.
Different forms of Gypsum have been found in ancient sculptures of Mesopotamia, Ancient Rome, and the Byzantine empire. At present, the United States, Brazil, and India are among the top 3 countries with the largest Gypsum reserves in the world.
Orbital pictures from the Mars Reconnaissance Orbiter (MRO) indicated the existence of Gypsum dunes in the extreme northern region of Mars.
Calcite with mottramite
Mohs hardness: 3
Absolute hardness: 9
Chemical formula: CaCO3
Calcite belongs to a carbonate group of minerals, and it is the most stable polymorph of calcium carbonate. Calcite is a common constituent of sedimentary rocks, most of which is formed from the dead marine organisms. One of the remarkable natural made Calcite structure has been found in the Snowy river cave in Lincoln County, New Mexico.
It has a Mohs hardness scale of 3 and a specific gravity of 2.71. So far, over 800 forms of calcite crystals have been identified. Most of them are transparent to visible light, though shades of yellow, brown, gray, black, orange, green, or violet can occur when it is charged with impurities.
The construction industry uses calcite in the form of marble and limestone to create cement and concrete. The chemical industry uses calcite as an acid neutralizer. And powered calcite (that has a white color) is often used as an inert coloring ingredient of paint.
Teal color fluorite with purple highlights | Wikimedia
Mohs hardness: 4
Absolute hardness: 21
Chemical formula: CaF2
The fluorite crystal is made of calcium and fluorine. Although the pure crystal is transparent, both in ultraviolet and visible light, impurities make it a colorful mineral. It exists as granular, massive, cubic, and octahedral crystals.
A large portion of this mineral occurs as vein fillings in rocks that have been subjected to hydrothermal activities. It is also found in the vugs and fractures of some limestones and dolomites. China, Mexico, and South Africa are amongst the largest fluorite producing countries in the world.
Fluorite is mostly used in the ceramics, metallurgical, and chemical industries. Due to its low dispersion property (which causes no or less chromatic aberration), it is also utilized in optical lenses.
Faceted blue apatite
Mohs hardness: 5
Absolute hardness: 48
Chemical formula: Ca5(PO4)3(F,Cl,OH)
Apatite is a set of phosphate minerals with similar physical properties and chemical compositions. It usually refers to fluorapatite, chlorapatite, and hydroxyapatite with high concentrations of F−, Cl−, and OH−, respectively.
Crystals with excellent color and clarity are often cut as faceted gemstones, while those with decent color and translucence are cut as cabochons.
The most common use of apatite is in fertilizer, i.e., for the production of phosphorous. It is also used to produce animal feed supplements, elemental phosphorous, phosphoric acid, and several phosphate compounds for the chemical industry.
More than 75 percent of the world’s reserves of phosphate rock are located in Western Sahara and Morocco. Its rock samples were also collected on Moon by astronauts during the Apollo program.
6. Orthoclase Feldspar
Orthoclase measuring 27*17 centimeter
Mohs hardness: 6
Absolute hardness: 72
Chemical formula: KAlSi3O8
Orthoclase Feldspar is polymorphous with minerals Sanidine and Microcline. It’s a common constituent of felsic igneous rocks and usually forms large crystals and masses in pegmatite.
Although most crystals of orthoclase are smaller than a few centimeters in length, the largest identified orthoclase crystal was over 30 feet long and weighed nearly 10,000 kilograms. It was discovered in a pegmatite in the Ural Mountains of Russia. It has been detected in igneous rocks found on the Moon and Mars.
Orthoclase is primarily used to manufacture glass and ceramics. One of the most popular orthoclase gems is Moonstone, a translucent to transparent material containing alternate layers of orthoclase and albite feldspar.
Rutilated quartz measuring 6.6*5.5*5.1 centimeter | Credit: Wikimedia
Mohs hardness: 7
Absolute hardness: 100
Chemical formula: SiO2
Quartz is the second most abundant mineral in the Earth’s crust after feldspar. This crystal is composed of silicon and oxygen atoms. It exists in two forms: α-quartz (trigonal) and β-quartz (hexagonal). The α- turns into β-quartz at 573 °C.
Quartz is resistant to both mechanical and chemical weathering. It occurs in almost every color; the most common of them are clear (transparent), white, black, green, pink, brown, gray, yellow, and purple.
While quartz’s color, luster, and diaphaneity make it valuable in the production of glass and as a gemstone, its heat resistance and electrical properties make it useful in electronic products. Another useful property of quartz is its crystals can vibrate at precise frequencies. Thus, they are used in a device called crystal oscillator.
Facet cut topaz gemstones in different colors
Mohs hardness: 8
Absolute hardness: 200
Chemical formula: Al2SiO4(F,OH)2
Topaz is a rare silicate mineral made of fluorine and aluminum. In its natural state, topaz is a golden brown to yellow. Different impurities and treatments make this crystal pale gray, pink, reddish-orange, wine red, or transparent/translucent.
Due to its hardness, various colors, and usual transparency, it is widely used in jewelry as a cut gemstone. Some topaz crystals are coated with metallic oxide to give it a multicolored iridescent luster. These crystals, called ‘mystic topaz,’ appear to change color when viewed from different angles under the bright light.
It is found in several locations worldwide where rocks like rhyolite and pegmatite are formed. Most of the fine-quality imperial topaz is created in the state of Minas Gerais in southeastern Brazil. Significant amounts of topaz are also found in the United States, Sri Lanka, Australia, Mexico, Japan, and Russia.
Five corundum crystals | Wikimedia
Mohs hardness: 9
Absolute hardness: 400
Chemical formula: Al2O3
Corundum is the naturally occurring aluminum oxide crystal that contains traces of titanium, vanadium, iron, and chromium. Since it is the third hardest mineral on the Mohs scale, it can scratch almost every other mineral.
Pure corundum is transparent; however, it can have different colors when impurities are present. Different colored corundum crystals have different names: the red colored corundum is known as ruby, while pink-orange is called padparadscha, and all others are called sapphire.
Due to its extreme hardness and density, it is used as an abrasive. Crushed corundum granules (of uniform size) are used for polishing compounds, grinding wheels and media, and various cutting applications.
It is one of the most popular gems: millions are sapphires and rubies are required every year to fulfill the demands of the jewelry market. Synthetic corundum is also a crucial component of several lasers. In fact, the first working laser developed in 1960 used a processed ruby crystal as the ‘gain medium’ to produce an intense burst of light.
Moissanite showed at 14x magnification | Credit: Brigham Young University Department of Geology in Utah
Mohs hardness: 9.5
Melting point: 2,730 °C
Chemical formula: SiC
Moissanite is a naturally occurring silicon carbide. It is rare tiny crystals found in some meteorites (originally formed as stardust) and kimberlites. It was first discovered by a French scientist Henri Moissan in 1893, who later won the Nobel Prize in Chemistry.
Natural moissanite is so rare that most of the moissanite crystals available today are laboratory-made. They are useful for industrial and commercial applications due to its hardness, thermal conductivity, and optical properties.
These properties make it a valuable material for high-pressure experiments, as a replacement for diamond. Since diamonds are very costly, moissanite crystals are often used in large-volume experiments.
Mohs hardness: 10
Absolute hardness: 1,500
Chemical formula: C
Diamond is the hardest known natural mineral according to the Mohs scale. Its hardness depends on its purity, and the hardest diamond can only be scratched by other diamonds. Some blue color diamonds are natural semiconductors, some are electrical insulators, and the rest are electric conductors.
It is entirely made of carbon atoms: each carbon atom is attached to four other carbon atoms through strong covalent bonds (the strongest type of chemical bond). It also has an extremely high density, ranging from 3,150 to 3,530 kg/m3.
Diamonds are formed at high pressures and temperatures that occur 100 miles below the Earth’s surface, although a few have come from as deep as 500 miles. While gem diamonds are suitable for jewelry and investment purpose, industrial diamonds are used in grinding, cutting, drilling, and polishing procedures.
About 140 million carats of diamonds are produced from mines every year. Russia has the largest diamond reserves, estimated at 650 million carats. Worldwide reserves are estimated to be 1.2 billion carats.
Recent studies have revealed a few crystals whose hardness is theoretically superior to that of cubic diamonds, according to computer simulations. The two most impressive of those materials are Wurtzite Boron nitride and Lonsdaleite.
Wurtzite Boron Nitride
Wurtzite Boron Nitride (wBN) has recently attracted much attention from both industrial and academic fields, due to its impressive properties and various potential applications. In addition to high hardness, it has high thermal conductivity, high thermal expansion, and good chemical stability.
A very small amount of wBN exists on Earth. They are either naturally found or manually synthesized. Naturally, these are produced during volcanic eruptions due to very high temperatures and pressure. Various simulations show that wBN can withstand 18 percent more stress than diamond.
Lonsdaleite, also known as the hexagonal diamond, was named in honor of Kathleen Lonsdale, a famous Irish crystallographic. It is a naturally occurring mineral that forms when meteorites containing graphite strike the Earth.
The heat and stress resulting from the strike transform the graphite into Lonsdaleite, which is similar to a diamond but arranged in a different shape. It has also been produced in the laboratory by heating and compressing graphite either via explosives or in a static press.
Simulations show that it has superior indentation strength (58 percent higher) than diamond. It can resist indentation pressure of 152 gigapascals, whereas diamond breaks at 97 gigapascals.