If you ask any random person what is the hardest material on Earth, the most likely answer would be diamond. It is fairly common knowledge. But is diamond actually the hardest material or mineral on Earth? We’ll find out in this article.
The hardness of any mineral is defined by its Mohs scale number: the harder the mineral, the higher its Mohs number. The Mohs scale was devised by a German geologist and mineralogist, Friedrich Mohs, in 1812.
Although the Mohs scale is not precise and strictly ordinal, it has a relevant use in geology, mainly to identify the hardness of various minerals. It works by analyzing the ability of a material to scratch the other (softer) material. 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 8 on the Mohs scale of hardness.
Below is our list of the hardest minerals in the world, ranked according to their Mohs hardness. We have also mentioned the absolute hardness of some of the minerals.
Mohs Hardness: 8
Topaz is a naturally occurring (silicate) mineral composed of fluorine and aluminum. In its natural state, topaz is mostly colorless, but it can also be found in golden brown and yellow color. Different impurities can make a topaz crystal appear pink, reddish, or golden-orange though they are extremely rare.
At Mohs hardness of 8, topaz is harder than some of the most well-known minerals such as quartz, tungsten, and zircon. But much softer than silicon carbide and diamond.
Due to its hardness, various colors, and usual transparency, it is widely used in jewelry as a cut gemstone.
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.
Other Material With Similar Mohs Hardness –
A drill bit made of hardened steel | Image Courtesy: Emrys2
Hardened steel is any carbon steel that has gone through a hardening process. In this process, high-carbon steel is first heated until it reaches its critical temperature before being quenched in water (or brine solutions).
The heat treatment and the subsequent quenching allow the steel to reach its maximum hardness but also make it very brittle and susceptible to breaking under stress. However, this issue can be resolved by a process called tempering.
In this process, the alloy is re-heated at much lower temperatures, then allowed to cool down gradually in still air. Tempering removes some of the excess hardness of the alloy but increases its elasticity.
As the name suggests, hardened steel is harder and much more ductile than ordinary steel (Mohs hardness 4.5).
The Mohs hardness of hardened steel can be used as a reference for harder minerals that we are going to discuss ahead.
Alexandrite under UV light | Image Courtesy: Wikimedia Commons
Mohs Hardness: 8.5
Chrysoberyl (“a gold-white spear”) is an exotic gemstone with the chemical formula BeAl2O4. At 8.5 Mohs hardness, it is one of the hardest minerals on Earth.
Chrysoberyl is often associated with another beryllium gemstone called beryl. The two minerals are composed of nearly similar elements – aluminum and beryllium, but they are present in different compositions (Chrysoberyl has a high ratio of aluminum to beryllium).
The mineral is found in three varieties – ordinary chrysoberyl, alexandrite, and cymophane.
Ordinary chrysoberyl is yellowish-green in color and has no special qualities. On the other hand, alexandrite and cymophane have unique qualities.
Alexandrite shows color changes under sunlight. This is due to the presence of small amounts of chromium ions in its crystal structure. It appears greenish in daylight and turns reddish under low light.
Chrysoberyl is known for its hardness and resistance to chemical weathering. It can be found in river sands and deposits with other minerals such as topaz, corundum, garnet, and diamond.
Other Minerals with Similar Mohs Hardness –
- Silicon Nitride
- Tantalum Carbide
Corundum crystals | Image Courtesy: Wikimedia Commons
Mohs Hardness: 9
Absolute Hardness: 400
Corundum is a naturally occurring aluminum oxide that contains traces of titanium, vanadium, iron, and chromium. Pure corundum is transparent and almost colorless; 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 pinkish-orange is called padparadscha, and all others are called sapphire.
Since corundum is very high up on the Mohs scale, it can scratch almost every other mineral. Due to its hardness and unusually high density, it is widely used as an abrasive. Crushed corundum granules (of uniform size) are used for polishing compounds, grinding wheels and media, and various cutting applications.
Corundum is also highly valued as a gemstone. Two of its gem varieties, ruby, and sapphire, are cardinal gems — they are traditionally considered more precious than other gems.
Other Minerals with Similar Mohs Hardness –
- Titanium carbide
- Tungsten carbide
- Titanium nitride
7. Moissanite (Silicon Carbide)
Moissanite, a crystal form of silicon carbide at 14x magnification
Mohs Hardness: 9.5
Moissanite is an extremely rare mineral and naturally occurring silicon carbide. It was first discovered in 1893 by French chemist and Nobel laureate Henry Moissan after whom the mineral is named.
At first, he mistakenly identified newly discovered moissanite crystals as diamonds due to their almost similar physical characteristics. Henry Moissan was able to discern these crystals as silicon carbide sometime in 1904. Interestingly, the first-ever artificial silicon carbide was synthesized a few years before this discovery.
The earliest known source of Moissanite on Earth were the rare C chondrites meteorites that contain high amounts of carbon and amino acids. These meteorites are known to carry some of the oldest minerals created in our solar system and even extrasolar contents.
It was not until the 1960s that the mineral was found in nature. One of the sites where Moissanite was discovered is a diamond mine in Russia’s Yakutia region.
Moissanite has several industrial and commercial uses due to its hardness, as well as, optical and thermal properties. Since moissanite is rarely available in nature, most of the moissanite in use today are artificially made in labs.
The mineral is mostly used as a diamond alternative. Instead of a diamond, you can buy moissanite jewelry which is cheaper than the ones made from real diamonds. It can also be used to conduct high-pressure experiments instead of diamonds.
Other Minerals with Similar Mohs Hardness –
- Boron Nitride
- Rhenium diboride
- Stishovite, and
- Titanium diboride
The Hope Diamond
Mohs Hardness: 10
Absolute Hardness: 1500
According to both Mohs and Vickers scales, diamond is the hardest natural mineral on Earth. It is a solid form of carbon with an extremely rigid arrangement of atoms.
It has a high refractive index and the highest thermal conductivity of any natural element. The density of diamonds ranges from 3,150 kg/m3 to 3,530 kg/m3.
Most gem-quality diamonds form at a depth of 100 miles below the Earth’s surface, where temperatures (and pressure) conditions are incredibly high. However, some rare diamonds have been known to come from below depths of 500 miles.
It is important to note that having the highest Mohs hardness doesn’t mean that diamonds are indestructible, as they can be scratched by other diamonds.
Apart from commercial uses such as jewelry, diamonds are widely used for industrial applications such as grinding, cutting, drilling, and polishing.
Diamonds have been produced synthetically in laboratories ever since their invention in the 1950s. Today, most synthetic (artificial diamonds) are produced by HPHT (high-pressure, high-temperature) process. Like natural diamonds, artificial ones can also be of different colors, which are obtained by adding various impurities such as nitrogen and boron during the diamond-making process.
More than 110 million carats of rough diamonds are mined every year.
5. Diamene (Graphene)
We all have heard about graphene at least once, but what exactly is it? In simple terms, graphene is a single layer of carbon atoms with a hexagonal lattice. It is perhaps one of the most remarkable materials developed in the 21st century.
Graphene is the base element in carbon nanotubes and has potential applications in multiple industries. Graphene is a perfect conductor of heat and electricity. For its size, it is the strongest known material on Earth.
But here we are talking about the hardest material (as we know that hardest and strongest are two different parameters). In terms of hardness and stiffness, we know that graphene is inferior to diamond. However, researchers have found a method to make graphene as hard as a diamond.
A team of scientists from the Advanced Science Research Center, City University of New York, have combined two layers of graphene sheets to create a new material that transforms into a diamond-like film upon compression, showing both elastic deformation and chemical changes. They named this new material ‘Diamene.’
In simple words, diamene can temporarily become stiffer and harder than diamond when compressed with enough force. It won’t be surprising to see this material in armor and protective clothing due to its flexibility and lightweight.
Interestingly, the research also concluded that adding more than two layers of graphene doesn’t make the material harder. Instead, it could have a reverse effect.
4. Wurtzite Boron Nitride
3D representation of Wurtzite structure
In recent years, Wurtzite Boron Nitride (w-BN) has attracted much attention, from both industrial and academic fields, due to its superior properties and various potential applications. In simple terms, it is a wurtzite form of the boron nitride in which nitrogen and boron atoms crystallize in six member ring formation.
Boron Nitride can be in amorphous (non-crystalline), hexagonal (similar to graphite), cubic (similar to diamond, but slightly weaker), and wurtzite form.
According to a study, wurtzite boron nitride is able to withstand 18 percent more stress than diamond. Wurtzite Boron also remains more stable in higher temperatures than the diamond, making it a better option for use in cutting and drilling tools.
In addition to extreme hardness, w-BN has high thermal conductivity and high thermal expansion. Due to excellent thermal and chemical stability, boron nitride ceramics are used in high-temperature equipment and metal casting.
Different from diamond and other important minerals, researchers still need to fully realize the potential (in terms of strength) of wurtzite boron nitride as it is extremely rare in nature. It is extremely difficult to produce pure w-BN artificially. Only small amounts of w-BN have been synthesized in a lab.
3. Palladium Microalloy Glass
Palladium is a rare silvery white metal with the lowest melting point of all platinum group metals (which include platinum, osmium, rhodium, ruthenium, and iridium).
While Palladium has a soft appearance and is ductile, its strength and hardness increase substantially under low temperatures.
Most of the world’s supply of palladium is used in the auto industry (catalytic converters) and the electronics sector. It is also used in medicine, water treatment, and printing process. The metal has numerous potential applications, but is constrained by its limited availability.
As we know, glasses and other ceramic materials are brittle, meaning they are strong but not so tough. The same can be said about reinforced glasses such as Gorilla Glass.
But in 2011, a team of researchers at Caltech came up with a specially designed metallic glass that is both stronger and tougher than most materials on Earth. The metallic glass is a microalloy of phosphorous, palladium, germanium, silicon, and silver.
Palladium allows the microalloy metallic glass to form shear bands (a narrow zone of intense strain), making the glass undergo intense plasticity under stress and thus causing it to bend rather than crack.
Palladium Microalloy Glass is much stronger than hardened steel and perhaps one of the hardest materials on this list that do not have carbon.
The material was developed by a collaboration of researchers at Caltech and Lawrence Berkeley National Laboratory under the U.S. Department of Energy.
Buckypaper | Wikimedia Commons
Imagine a material with one-tenth the weight but at least 250 times stronger than steel and can also conduct electricity. Its potential applications would be endless. The material we are referring to is called ‘Buckypaper,’ which first gained attention from the scientific community in the early 2000s.
Buckypaper is made of carbon nanotubes which are about 50,000 times thinner than our hair ( between 25-µm to 72-µm thick). A carbon nanotube (CNT) is basically a cylindrical tube made up of graphene with a diameter of a nanometer. It is an allotrope of carbon.
There are basically two main variants of nanotubes – Single-wall carbon nanotubes (SWCNTs) and Multi-wall carbon nanotubes (MWCNTs). They are one of the strongest materials ever synthesized.
The powerful atomic bonds of buckypaper make it twice as hard as diamond and 250 times hard than steel (ordinary steel has Mohs hardness between 4-4.5).
Buckypaper has some other intriguing characteristics that separate it from ordinary composite materials. For example, it conducts electricity and disperses heat efficiently, which makes it a feasible material for use in the semiconductor and electronics industry. Buckypaper also has potential biological applications.
Although it may sound like science fiction, this remarkable material is light and stable enough to potentially be used for futuristic structures like a space elevator.
On the left-hand side (a) is the pure diamond, and on the right (b) is a diamond with lonsdaleite impurities. Two diamond samples from the Popigai crater in Siberia. | Image Courtesy: Hiroaki Ohfuji et al./Nature
Lonsdaleite, also known as the hexagonal diamond, is a rare allotrope of carbon (like graphite and diamond) that is only found in meteorite debris in nature.
Lonsdaleite was first discovered in 1967 at the Canyon Diablo meteorite, an asteroid that created the Barringer Crater in Arizona, U.S. It is named after British crystallographer Kathleen Lonsdale widely known for her work in X-ray crystallography.
Lonsdaleite is called a ‘hexagonal diamond‘ due to its hexagonal crystal structure, which is similar to that of wurtzite boron nitride.
Imagine a carbon-rich meteorite containing graphite hurtling down toward the Earth. Upon collision with the Earth’s surface, the meteorite experience a massive surge in temperature and pressure. This increased pressure and temperature allow the graphite to compress and transform into a diamond.
However, instead of gaining a cubic lattice, which is found in ordinary diamonds, it retains the hexagonal lattice of graphite. In simple terms, lonsdaleite is similar to diamond but with a different crystal structure.
According to computer simulations, lonsdaleite is estimated to be 58 percent harder than diamond and can withstand higher pressures (indentation hardness).
However, most lonsdaleite found in nature exhibits lower hardness than diamonds. This is likely due to the impurities present in the material. Researchers believe that impurity-free lonsdaleite would be much harder than any diamond found on Earth.
More To Know
Is There Any Other Method To Measure the Hardness of Materials?
In material science, there are mainly three ways one can measure the hardness of a mineral/material – scratch, indentation, and rebound.
The scratch hardness, as we talked about earlier, measures how resistant a material is from a fracture due to friction from a harder material. Perhaps the most common scratch hardness test is the Mohs scale. To perform a scratch test, mineralogists often use an instrument called a sclerometer.
Indentation hardness tests determine the hardness of a material by measuring its resistance to deformation from a sharp object (indenter). It is mostly used in mechanical engineering. The two most common indentation hardness test scales are – the Brinell scale and the Vickers hardness test.
Brinell Scale – Established in 1900 by Swedish engineer John Brinell, the Brinell scale was the first-ever standardized test for indentation hardness. It is denoted by BHN or HB.
Typically, a 10 mm diameter steel ball is used as an indenter to perform the test. However, for harder materials, the steel ball is replaced by a tungsten carbide ball. The maximum force used in this test is 3,000 kgf.
Vickers Hardness Test – An alternative to the Brinell scale, the Vickers hardness test was developed at a British engineering company Vickers Ltd in 1921.
Diamond indenter used in a Vickers test | Image Courtesy: R. Tanaka
Vickers test is often considered the ‘easiest’ hardness test to perform since the size and type of the indenter don’t affect the required calculations. An indenter can be used for all materials/metals irrespective of their hardness. It is denoted by Vickers Pyramid Number (HV).
The third type of hardness test is called the rebound hardness test. In this test, a diamond-tipped ball is dropped on the test material from a fixed height to measure its hardness.
Leeb rebound hardness test – One of the commonly used methods for testing metal hardness is the Leeb rebound hardness test which uses the rebound velocities to determine the hardness.