Hvad er en diamant?

Introduction

A diamond is a mineral composed of a single element: carbon. The same element that forms graphite — the soft, dark material in a pencil — creates the hardest known natural substance when its atoms arrange themselves under extreme pressure and temperature deep within the Earth.

That apparent contradiction is the first thing worth understanding about diamonds. Their extraordinary properties — hardness, brilliance, fire, and durability — are not mysterious. They follow directly from how carbon atoms bond to one another in the diamond crystal. Once you understand the structure, everything else about how a diamond looks and behaves makes sense.

This article covers what a diamond actually is at the atomic level, why it handles light the way it does, and what physical properties make it unique among gemstones. It is the foundation for everything else in this encyclopedia.

Key Concepts

Before reading further, three terms will appear throughout this article and across the encyclopedia:

• Brilliance — the return of white light from within the diamond back to the observer's eye. A well-cut diamond appears bright because it reflects most of the light that enters it.
• Fire — the separation of white light into its spectral colours (red, orange, yellow, green, blue, violet) as it passes through the stone. Fire is what produces flashes of colour when a diamond moves.
• Hardness — resistance to scratching. Diamond ranks 10 on the Mohs scale, the maximum. No natural material can scratch a diamond except another diamond.

These properties are not coincidental. Each one traces back to the atomic structure described below.

Crystal Structure and Carbon Bonding

Carbon: One Element, Many Forms

Carbon is the sixth element in the periodic table. It is remarkably versatile — it forms more compounds than any other element, and it exists in several pure crystalline forms called allotropes. The two most familiar carbon allotropes are graphite and diamond.

In graphite, carbon atoms bond in flat sheets. Each atom connects to three neighbours in a plane, forming hexagonal rings. The sheets themselves are held together only by weak forces, which is why graphite is soft and slippery — the layers slide past each other easily. This is what makes a pencil write.

In diamond, the arrangement is fundamentally different. Every carbon atom bonds to four neighbours in a three-dimensional tetrahedral pattern. This bonding geometry is called sp3 hybridisation. Each bond is a strong covalent bond — a direct sharing of electrons between atoms — and every bond in the structure is identical in strength and length (1.54 angstroms).

The Diamond Lattice

The result is a face-centred cubic crystal lattice — a continuous, three-dimensional network where every atom is locked into place by four equally strong bonds radiating outward at angles of 109.5°. There are no weak planes, no layers that slide, no gaps in the bonding.

This is why diamond is so hard. To scratch a diamond, you would need to break these covalent bonds. Since the entire crystal is one interconnected network of identical strong bonds, there is no easy path for fracture. The hardness is not a surface property — it is a consequence of the bonding extending uniformly in all directions through the entire stone.

The same structure also explains diamond's density (3.52 g/cm³), its thermal conductivity (roughly five times higher than copper), and its chemical inertness. The atoms are packed tightly and bonded securely. Very little can disrupt the arrangement.

Why Pressure and Temperature Matter

Natural diamonds form at depths of approximately 150 to 200 kilometres below the Earth's surface, where pressures exceed 5 gigapascals and temperatures range from 900 to 1,300 °C. These conditions provide the energy needed to force carbon atoms into the dense sp3 bonding arrangement rather than the looser graphite structure.

At the Earth's surface, graphite is actually the more thermodynamically stable form of carbon. Diamond persists because the energy barrier to rearranging its bonds is enormous — the atoms are locked in place, and without extreme conditions, they stay there. A diamond on your finger is technically metastable, but on any human timescale, it is permanent.

Physical and Optical Properties

Refractive Index

When light passes from air into a denser material, it slows down and changes direction — a phenomenon called refraction. The degree of bending is measured by the refractive index (RI). Diamond's refractive index is 2.417, which is exceptionally high for a natural gemstone.

For comparison:

A high refractive index means light bends sharply when entering the stone. In a well-cut diamond, this bending directs light back out through the top of the stone rather than leaking out through the sides or bottom. The result is brilliance — that characteristic brightness that makes a diamond appear to glow from within.

Diamond's RI also creates a low critical angle (approximately 24.4°), meaning light striking internal surfaces at even modest angles undergoes total internal reflection. This is why cut proportions matter so much: a well-angled pavilion bounces light back through the crown and table. A poorly angled one lets it escape.

Dispersion

Dispersion is the separation of white light into its component wavelengths — the same effect that produces a rainbow when sunlight passes through a glass prism. Diamond's dispersion coefficient is 0.044, measured as the difference in refractive index between red light (686.7 nm) and violet light (430.8 nm).

This is a strong dispersion value. It means that when white light enters a diamond, the violet wavelengths bend more than the red, and by the time the light exits the stone, the colours have separated enough to be visible. These flashes of spectral colour — appearing and disappearing as the diamond, light source, or observer moves — are what gemologists call fire.

Not every diamond displays equal fire. Cut geometry controls how much dispersion the eye actually sees. Steep crown angles and smaller table facets tend to increase visible fire. Fire is most apparent under spot lighting or direct sunlight, and less visible under diffused fluorescent light.

Lustre

Diamond's surface reflects light with a distinctive quality called adamantine lustre — from "adamas," the Greek word for unconquerable, which also gave diamond its name. This is the sharp, bright surface reflection you see before light even enters the stone. It results from the high refractive index: the greater the difference in RI between a material and the surrounding air, the more light reflects off the surface.

Adamantine lustre is specific to a small number of minerals. Most gemstones display vitreous (glass-like) lustre. Diamond's surface reflection is noticeably sharper and brighter, contributing to its visual impact even before brilliance and fire come into play.

Hardness and Durability

Diamond is the hardest known natural material — 10 on the Mohs scale. But the Mohs scale is ordinal, not proportional. The gap between diamond (10) and corundum (9, the mineral family of sapphires and rubies) is far larger than the gap between corundum and any lower-ranked mineral. In absolute terms, diamond is roughly four times harder than corundum.

This hardness makes diamond exceptionally resistant to scratching. In practical terms, a diamond ring worn daily for decades will maintain its polish while virtually every other gemstone will accumulate surface abrasion over time.

However, hardness is not the same as toughness. Diamond has perfect cleavage along four planes corresponding to the octahedral faces of its crystal structure. A sharp impact along a cleavage direction can split a diamond. This is how diamond cutters have shaped rough diamonds for centuries — by exploiting these planes. It also means a diamond is not indestructible. It can chip at thin edges like girdles and pointed tips if struck at the right angle.

See Hardness vs Toughness for the full discussion.

Frequently Asked Questions

What are diamonds made of?

Diamonds are made entirely of carbon atoms arranged in a rigid three-dimensional lattice structure. Each carbon atom bonds to four neighbouring atoms through strong covalent bonds in a tetrahedral pattern called sp3 hybridisation, creating the hardest known natural material.

Why do diamonds sparkle so much?

Diamonds sparkle because of their exceptionally high refractive index (2.417), which bends light sharply and reflects it back out through the top of the stone as brilliance. Their strong dispersion (0.044) also splits white light into spectral colours, producing the flashes of fire you see when the diamond moves.

How hard is a diamond compared to other gemstones?

Diamond is the hardest natural material, rated 10 on the Mohs hardness scale. It is roughly four times harder than corundum (sapphire and ruby) at Mohs 9, and no natural material can scratch a diamond except another diamond.

What is the difference between diamond and graphite?

Both diamond and graphite are made of pure carbon, but their atoms are arranged differently. In diamond, every carbon atom bonds to four neighbours in a 3D lattice, making it extremely hard. In graphite, carbon atoms bond in flat sheets held together by weak forces, making it soft and slippery.

Can a diamond break even though it is the hardest material?

Yes, hardness (scratch resistance) is not the same as toughness (resistance to breaking). Diamond has perfect cleavage along four planes, meaning a sharp impact at the right angle can chip or split it. This is how diamond cutters have shaped rough stones for centuries.

Summary

A diamond is crystalline carbon — every atom bonded to four neighbours in a continuous, three-dimensional lattice of identical covalent bonds. This structure, formed under immense pressure deep within the Earth, produces the hardest natural material known and gives diamond its exceptional optical properties: a high refractive index that creates brilliance, strong dispersion that produces fire, and adamantine lustre that makes its surface shine with a distinctive sharpness. Understanding what a diamond is at the atomic level is the foundation for understanding everything else — how it is graded, why cut matters, and what makes one stone perform better than another.