Diamond: The World's Hardest Natural Material

Diamond is a solid form of pure carbon with its atoms arranged in a crystal structure, called diamond cubic. Known for its unmatched hardness and brilliant optical properties, it serves as both a luxury gemstone and a critical tool in heavy industry. From the depths of the Earth's mantle to the vacuum of space, diamonds represent one of the most fascinating intersections of chemistry, geology, and economics.

At its most basic level, a diamond is a native mineral consisting entirely of carbon (C). Its unique properties arise from the way carbon atoms bond, creating a rigid, three-dimensional lattice that resists deformation and scratching better than any other natural substance.

Diamond unit cell, showing the tetrahedral structure

Key Facts

Main diamond producing countries (in French)

Chemical Composition: Pure carbon (C).
Hardness: 10 on the Mohs scale, the highest possible rating.
Crystal System: Cubic (Hexoctahedral).
Luster: Adamantine (a brilliant, diamond-like shine).
Refractive Index: 2.418 at 500 nm.
Formation: Primarily formed under extreme pressure and temperature in the Earth's mantle.

Physical and Chemical Properties

Theoretically predicted phase diagram of carbon, from 1989 and updated with newer work[19]

Mechanical Strength and Hardness

Diamond is the defining mineral for the top of the Mohs scale, meaning it can scratch any other natural mineral. This extreme hardness is due to the tetrahedral structure where each carbon atom is covalently bonded to four others. While incredibly hard, diamonds possess perfect cleavage in four directions (111), meaning they can be split along specific planes if struck correctly.

The extreme hardness of diamond in certain orientations makes it useful in materials science, as in this pyramidal diamond embedded in the working surface of a Vickers hardness tester.

Optical and Thermal Characteristics

The high refractive index of diamond allows it to bend light significantly, contributing to its characteristic "fire" or dispersion. Most diamonds are colorless, but impurities or structural defects can produce a variety of colors, including blue, pink, and brown.

Picture of a diamond — The most famous colored diamond, the Hope Diamond

Beyond aesthetics, diamonds possess exceptional thermal conductivity, allowing them to dissipate heat more efficiently than almost any other material. They are also chemically stable, though they can be flammable under specific high-temperature conditions in oxygen-rich environments.

A museum display of jewelry items. Three brooches each consist of a large brown central gem surrounded by many clear small stones. A necklace has a large brown gem at its bottom and its string is all covered with small clear gems. A cluster-shaped decoration contains many brown gems. — Brown diamonds at the National Museum of Natural History in Washington, D.C.

Geology and Formation

A triangular facet of a crystal having triangular etch pits with the largest having a base length of about 0.2 millimetres (0.0079 in) — One face of an uncut octahedral diamond, showing trigons (of positive and negative relief) formed by natural chemical etching

Origin in the Mantle

Most natural diamonds form in the Earth's mantle, far below the crust, where pressures and temperatures are sufficient to crystallize carbon. They are often found in cratons—the ancient, stable cores of continental plates.

Geologic provinces of the world. The pink and orange areas are shields and platforms, which together constitute cratons.

These crystals are transported to the surface via deep-seated volcanic eruptions. The resulting structures are known as volcanic pipes (or kimberlite pipes), which act as the primary source for diamond mining.

Inclusions and Age

Diamonds often trap other minerals during their growth, known as inclusions. For example, red garnet inclusions can provide clues about the pressure and temperature conditions of the mantle at the time of formation. These inclusions also help geologists determine the age zones of the diamond.

Extraterrestrial Diamonds

Carbon crystallization is not limited to Earth. Scientific evidence suggests that diamonds may form in space, and theoretical models propose that "diamond rain" may occur on ice giants like Uranus and Neptune due to the crushing of methane.

Industrial and Commercial Use

Necklace of rough light brown diamonds under UV light (top) and normal light (bottom)

Gem-Grade Diamonds

Gem-quality diamonds are valued based on their cut, color, clarity, and carat weight. The process of turning a rough octahedral crystal into a faceted gem requires precision polishing to maximize light reflection.

A clear faceted gem supported in four clamps attached to a wedding ring — A round brilliant cut diamond set in a ring

Industrial-Grade Diamonds

Because of their hardness, industrial diamonds are indispensable for cutting, grinding, and drilling. They are embedded into metal blades and drill bits to penetrate hard rock or steel.

A polished metal blade embedded with small diamonds — Close-up photograph of an angle grinder blade with tiny diamonds shown embedded in the metal

Synthetics and Simulants

Modern technology allows for the creation of synthetic diamonds using two primary methods: High-Pressure High-Temperature (HPHT) and Chemical Vapor Deposition (CVD). These are chemically and physically identical to natural diamonds. In contrast, simulants (like cubic zirconia or silicon carbide) look like diamonds but have different chemical compositions.

A round sparkling, clear gemstone with many facets. — Gem-cut synthetic silicon carbide set in a ring

Summary of Diamond Specifications

Diavik Mine, on an island in Lac de Gras in northern Canada

Property	Value/Description
Formula Mass	12.01 g/mol
Crystal Habit	Octahedral
Specific Gravity	3.52 ± 0.01
Density	3.5–3.53 g/cm³
Dispersion	0.044
Luster	Adamantine

Frequently Asked Questions

Eclogite with centimeter-size garnet crystals

A large rectangular pink multifaceted gemstone, set in a decorative surround. The decoration includes a row of small clear faceted gemstones around the main gem's perimeter, and clusters of gems forming a crest on one side. The crest comprises a three-pointed crown faced by two unidentifiable animals. — The Daria-i-Noor Diamond—an example of unusual diamond cut and jewelry arrangement.

Diamond balance scale 0.01–25 carat jeweler's measuring tool

A diamond scalpel consisting of a yellow diamond blade attached to a pen-shaped holder — A scalpel with synthetic diamond blade

A diamond knife blade used for cutting ultrathin sections (typically 70 to 350 nm) for transmission electron microscopy

A clear octahedral stone protrudes from a black rock — The slightly misshapen octahedral shape of this rough diamond crystal in matrix is typical of the mineral. Its lustrous faces also indicate that this crystal is from a primary deposit

Cathodoluminescence image of a diamond, taken in a scanning electron microscope

Do diamonds form from coal?

No. This is a common misconception. Most diamonds formed billions of years ago, long before land plants existed to create coal. They form from carbon trapped deep in the Earth's mantle.

What is the difference between a synthetic diamond and a simulant?

A synthetic diamond is chemically identical to a natural diamond, created in a lab. A simulant is a different material entirely (such as moissanite or cubic zirconia) that only mimics the appearance of a diamond.

Can a diamond be destroyed?

While diamonds are the hardest natural substance (resistant to scratching), they are not indestructible. They can be shattered with a hammer due to their cleavage planes, and they can burn if exposed to extreme heat in the presence of oxygen.

What causes diamonds to have different colors?

Color is usually caused by impurities. For example, nitrogen can create yellow tones, while boron creates blue diamonds. Some colors result from structural defects in the crystal lattice rather than chemical impurities.