Diamond has rightfully earned its place as the hardest mineral on Earth, with a perfect 10 rating on the Mohs hardness scale. Yet a few rare minerals can rival diamond in hardness, such as the lesser-known moissanite, which earns a 9.25 on the same scale. Even more impressive is the ultra-durable mineral wurtzite boron nitride, able to withstand a whopping 18% more stress than diamond before deforming. When it comes to prized gemstones, ruby and sapphire take the prize as the hardest after diamond, both rating 9 on Mohs’ scale.
As for rocks, the highest honors for hardness go to metamorphic rocks. Subjected to enormous heat and pressure beneath the Earth’s surface, metamorphic rocks recrystallize with a high concentration of durable minerals tightly interlocked like pieces of a puzzle. This dense crystalline structure makes metamorphic rocks incredibly hard and resistant to scratching. Quartzite, as an example, commonly rates higher on hardness than igneous rocks like granite.
While measurements like the Mohs scale can indicate hardness, a rock’s true “toughness” is better gauged by compressive strength—its ability to resist breaking under pressure. By this measure, rocks like flint and chert are among the toughest, which helps explain their use in early human tools and arrowheads. Toughness, not just hardness, is key for rocks used in construction and other heavy-duty applications.
Mohs Scale of Mineral Hardness
To determine and compare the relative hardness of minerals, geologists use the Mohs hardness scale. This scale was developed in 1812 by the German mineralogist Friedrich Mohs, who selected 10 readily available minerals and arranged them in order of increasing hardness. The softest mineral, talc, is given a value of 1, while the hardest mineral, diamond, is rated at 10.
To use the Mohs scale, the test mineral is scratched against each of the reference minerals, starting with talc, until a scratch is made. The hardness value of the mineral that scratches the test mineral is then recorded as the hardness number. For example, apatite (hardness 5) will scratch orthoclase (hardness 6), but orthoclase will not scratch apatite.
Here are the 10 standard minerals of the Mohs hardness scale, from softest to hardest:
Over time, additional minerals have been informally inserted into the Mohs scale according to their observed scratch resistance between the reference points. For example, the gemstone chrysoberyl is considered to have a hardness of 8.5, midway between corundum and diamond.
While useful for basic comparisons, the Mohs scale has limitations. It is strictly qualitative rather than precisely quantitative. And it only indicates hardness under a static load rather than dynamic stress and impact. Alternative scales have been developed to address these issues, but Mohs remains the most widely recognized hardness scale in mineralogy.
The Hardest Minerals
Diamond earns its perfect 10 rating on Mohs’ scale for good reason. Each carbon atom in diamond is tetrahedrally bonded to four other carbons in an incredibly stable and rigid crystalline structure. This accounts for diamond’s tremendous hardness and resistance to scratching and cutting. In fact, diamond is the only material hard enough to cut other diamonds.
But while diamond is extraordinarily hard, it is not very tough. Hardness indicates resistance to localized deformation, while toughness indicates resistance to fracture. Diamond has four directional cleavage planes along which it can break if impacted in specific orientations.
Diamond remains the undisputed champion in terms of hardness, but a few other rare minerals have been found to exceed it in toughness.
Discovered in 1893 by Nobel Prize winner Henri Moissan, moissanite is silicon carbide, a compound of silicon and carbon. Its hardness of 9.25 on the Mohs scale makes it the second hardest mineral after diamond. With greater toughness than diamond, moissanite is seeing increased use as a diamond simulant in jewelry.
Boron nitride is a human-made ceramic compound structurally similar to carbon diamonds, with alternate boron and nitrogen atoms forming tetrahedrons. The mineral wurtzite boron nitride, one of the harder forms, has been produced in labs and is estimated to be 18% harder than diamond, according to atomistic models and simulations.
Fullerites are spherical or elliptical cage-like molecules of carbon named after Buckminster Fuller’s geodesic dome designs. The fullerite C60, made of 60 carbon atoms arranged in interconnected hexagons and pentagons, is known as a buckyball. Calculations suggest C60 fullerite could be up to 30% harder than diamond.
A hexagonal variant of diamond called lonsdaleite (also known as hexagonal diamond) is believed to be 58% harder than cubic diamond based on computational simulations and limited experimental evidence. Lonsdaleite forms naturally from meteorite impacts and its rarity has prevented thorough testing.
Beta Carbon Nitride
Predicted through computational modeling, beta carbon nitride is thought to possibly be the hardest known material, up to 2.5 times harder than diamond. But despite decades of research, it has yet to be definitively synthesized in the lab. The hypothetical material continues to tantalize scientists seeking superhard crystals.
Other Very Hard Minerals
Aside from the minerals harder or nearly as hard as diamond, many others still qualify as very hard, rating 8 and above on Mohs’ scale.
Corundum is crystallized aluminum oxide often found as transparent gemstones. Ruby and sapphire are gem-quality corundum, colored by traces of chromium and iron respectively. With a Mohs hardness of 9, corundum is highly scratch resistant and only diamond can cut it.
The beryllium aluminum oxide mineral chrysoberyl is prized for its hardness, transparency, and luster. While ordinary chrysoberyl rates 8.5 on the scale, the cat’s eye variety known as cymophane rates a slightly lower 7.5-8 due to fibrous inclusions.
Topaz is an aluminum silicate mineral that can form enormous colorless crystals. Large high-clarity specimens are valued as gems. Topaz has a Mohs hardness of 8, making it exceptionally scratch resistant.
Nephrite is a variety of jade consisting of intricately intergrown amphibole minerals. Though relatively soft at 6-6.5 Mohs hardness, nephrite is one of the toughest gems, highly resistant to breakage and chipping. Jade was used for axes and other early tools, taking advantage of its toughness.
Comparison Table of the Hardest Known Minerals
|– Hardest known mineral
– Pure crystalline carbon
– Very high hardness but lower toughness
|– Natural silicon carbide mineral
– Nearly as hard as diamond
– More toughness than diamond
|– Aluminum oxide
– Ruby and sapphire varieties
– Highly scratch-resistant
|– Beryllium aluminum oxide
– Transparent and lustrous
– Extremely hard gemstone
|– Aluminum silicate
– Forms large colorless crystals
– Very scratch-resistant
|Cubic Boron Nitride
|– Synthetic, second hardest after diamond
– Excellent abrasive, can polish diamonds
|Wurtzite Boron Nitride
|– 18% harder than diamond in simulations
– Hexagonal crystal structure
|– Hexagonal polymorph of diamond
– Potentially 58% harder than diamond
|– Cage-like carbon molecule
– Possibly 30% harder than diamond
|– Hypothetical, predicted to be much harder than diamond
The Hardest Rocks
When it comes to rocks rather than individual minerals, the hardest tend to be certain metamorphic rocks containing strongly interlocking mineral crystals. The immense heat and pressure metamorphic rocks endure causes them to recrystallize into dense non-porous forms with few weaknesses.
Quartzite begins as quartz sandstone before undergoing metamorphosis. quartz grains fused together form an extremely hard, compact quartzite. With a Mohs hardness of 7, quartzite is harder than granite (6) and resistant to scratching. Its toughness allows it to withstand conditions like blasting and weathering.
Kimberlite is a volcanic igneous rock that originates deep in the mantle, entraining diamonds on its way up. The magma cools into an extremely hard matrix of olivine, serpentine, and other minerals intergrown with the diamond crystals. Kimberlite erodes very slowly due to its hardness.
Lamproite is an ultrapotassic igneous rock also containing diamonds, Indicating an origin in the mantle. Lamproites have a molasses-like texture but gain strength through a framework of rigid mineral crystals. This makes lamproite highly resistant to fragmentation and abrasion.
Novaculite is a dense siliceous sedimentary rock formed from deep ocean chert. The enormous pressure compacted microscopic quartz grains into an incredibly hard rock used since ancient times for making stone tools and sharpening blades. Novaculite’s Mohs hardness exceeds 7.
Gneiss is a banded metamorphic rock with alternating lighter and darker mineral layers. Subjected to intense heat and pressure, the minerals recrystallize and realign. Gneiss can thus reach Mohs hardnesses of 7 or higher. Its interlocked mineral grains give gneiss both hardness and toughness.
Comparison Table of the Hardest Known Rocks
|– Metamorphosed sandstone
– Compacted quartz grains
– Harder than granite
|– Metamorphosed chert
– Microscopic quartz grains
– Used for stone tools
|– Igneous, volcanic rock
– Contains diamonds
– Very erosion resistant
|– Igneous rock from the mantle
– Contains diamonds
– Highly abrasion resistant
|– Metamorphic, banded rock
– Tightly interlocked minerals
– Hard and tough
|– Metamorphosed coal
– High carbon content
– Very dense and hard
|– Sedimentary rock
– Compacted quartz pebbles
– Durable construction material
|– Igneous, granitic rock
– Coarse-grained crystals
– Tough and resistant
|– Metamorphosed limestone
– Recrystallized calcite grains
– Prone to etching/dissolving
While Mohs hardness values are useful for comparing scratch resistance, a mineral or rock’s true durability depends heavily on its toughness—its ability to resist fracturing and shattering. Toughness indicates how much force or impact a material can withstand without breaking.
Unfortunately, quantifying toughness is much more difficult than measuring hardness. Toughness is affected by many variables like crystal structure, bonding forces, and internal flaws. There is no universally accepted scale for rating toughness. Nonetheless, scientists have developed ways to gauge the toughness of minerals based on quantitative measurements.
Fracture toughness indicates how well a material tolerates the propagation of cracks when loaded. It is measured in units of MPa·m^1⁄2 or ksi·in^1⁄2. Materials with higher values are generally tougher. Fracture toughness accounts for both inherent material properties and crack-tip stresses.
Impact resistance or impact strength defines how much mechanical shock or blows a mineral can withstand before fracturing. It is measured by tests like repeatedly dropping weighted projectiles onto a specimen from increasing heights until breakage. This dynamic test assesses toughness under sudden impacts.
Dividing a mineral’s hardness value on the Mohs scale by its toughness level gives a quantitative hardness-to-toughness ratio. The higher this ratio, the more prone a material is to shatter upon a sudden sharp blow instead of deforming temporarily. Very hard but brittle minerals like diamond have high ratios.
Unique Uses for Hard Rocks and Minerals
The incredible hardness and toughness of certain rocks and minerals lends them to specialized industrial and commercial uses not practical with softer substances.
Diamond is renowned for its use in cutting, drilling, and polishing tools. Diamond blades and drill bits harness diamond’s hardness to penetrate the toughest materials. Diamond powder suspended in liquid or paste abrades away materials as hard as granite, quartz, and concrete.
After diamond, corundum is the most commonly used abrasive. Ruby and sapphire grains bonded to grinding wheels, sandpaper, and machining tools sharpen, smooth, and polish metal alloys, plastics, and hard woods.
Quartzite Building Stone
With compressive strengths exceeding 15,000 psi, quartzite is ideally suited for use as crushed stone aggregate, railway ballast, and road fill. Quartzite cladding on buildings resists weathering and acid rain degradation. The durability of quartzite makes it a prominent dimension stone.
Nephrite jade is treasured in Chinese culture for carving ornamental objects like bowls, figurines, jewelry, and more. Beyond its beauty, nephrite jade’s extreme toughness allows it to be carved into thin, delicate shapes that would shatter if attempted with harder but more brittle minerals.
Gems like diamond, sapphire, and topaz are popular in jewelry precisely because their Mohs hardness values of 8 to 10 resist abrasion and scratching during everyday wear. Their beauty endures because of hardness. Toughness is also vital – a hard but fragile gem prone to chipping or cracking has limited use in jewelry.
Frequently Asked Questions about the Hardest Rocks and Minerals
Which is the hardest known mineral?
Diamond is universally recognized as the hardest mineral, rating 10 on the Mohs hardness scale. The tetrahedral carbon bonds in diamond’s crystal lattice give it incredible hardness.
Are there any minerals harder than a diamond?
A few ultra-rare minerals like lonsdaleite and wurtzite boron nitride have been shown in simulations and limited tests to potentially exceed diamond in hardness. But they have yet to be definitively proven harder through rigorous testing.
What is the toughest mineral?
Toughness and hardness are different properties. The toughest gems include jadeite and nephrite jade. Their tightly intergrown crystals resist fracturing, chipping, and breaking under impact. But they are softer than diamonds.
What is the hardest rock type?
Metamorphic rocks tend to be the hardest rocks overall. Their high pressures and temperatures recrystallize the minerals into dense, tightly bonded forms. Quartzite is one example that can be harder than granite.
Can you scratch diamond with other materials?
Only a select few ultra-hard substances like cubic boron nitride are capable of scratching diamond. Diamond is able to cut any material, including other diamonds.
How is a mineral’s hardness measured?
The Mohs scale ranks minerals from 1 (softest) to 10 (diamond) based on their ability to scratch softer minerals. It is the most widely used qualitative hardness assessment technique.
Why are abrasives made from hard minerals?
Very hard minerals like diamond and corundum create excellent abrasives for cutting, grinding, and polishing tools. Their hardness allows them to finely wear away softer materials by scratching their surface.
Are emeralds and rubies very hard stones?
Yes, rubies and emeralds are varieties of the mineral corundum, which is very hard at 9 on Mohs’ scale. Only diamond is harder. Their hardness makes them durable jewelry gems.
Can a rock be harder than a diamond?
While individual rock minerals do not exceed diamond in hardness, some metamorphic rocks can effectively be harder than diamond when their tough interlocked crystals are considered together.
When surveying the natural mineral kingdom, diamond stands supreme in hardness, but a rare few challengers exist like hypothetical beta carbon nitride. For toughness, other gems like jadeite and nephrite rate more favorably. Metamorphic rocks tend to boast the highest hardness and durability among rocks. From abrasives to building materials to gemstones, the incredible properties of the hardest rocks and minerals grant them a valued place in human culture and technology.