Diamond is pure carbon arranged in a tight, three-dimensional cubic lattice, with every carbon atom bonded to four neighbors. Those strong, evenly spaced bonds make diamond the hardest natural material and give it its high refractive index. Graphite is also pure carbon, but its atoms sit in weakly linked sheets, which is why one is a gemstone and the other is pencil lead. Small amounts of nitrogen or boron in the lattice decide a diamond's type and much of its color.
A crystal made of one element
Most gemstones are compounds of several elements. Ruby is aluminum oxide, emerald is a beryllium aluminum silicate, and garnets mix several metals with silicon and oxygen. Diamond contains only carbon, apart from trace impurities measured in parts per million.
In the diamond structure, each carbon atom sits at the center of a tetrahedron, joined to four other carbon atoms at its corners. The atoms share electrons in strong covalent bonds, and chemists call this bonding arrangement sp3. The bond angle is about 109.5 degrees, and the distance between atoms is about 0.154 nanometers. That pattern repeats in every direction to build a cubic crystal, which is why diamond belongs to the cubic (isometric) system described on the crystal systems page.
The common crystal shape of natural diamond is the octahedron, with eight triangular faces. Cubes, dodecahedrons and flattened twinned crystals called macles also occur. The crystal forms and twinning page shows these shapes.
Why diamond is so hard
Hardness depends on how well a material resists scratching, and that comes down to bond strength and how densely bonds pack together. Diamond has short, strong carbon-carbon bonds running in all three dimensions, with no weak direction for a scratching point to exploit. It sits at 10 on the Mohs hardness scale, and on absolute scales it is several times harder than corundum, the mineral at 9.
Hardness is not the same as toughness. Diamond has perfect cleavage parallel to its octahedral faces, in four directions, where the atomic planes are spaced farthest apart. A sharp blow along one of these planes can split a diamond. Cutters have used this property for centuries to divide rough crystals, and it is also why a diamond can chip at a thin girdle or a pointed corner.
Diamond and graphite
Graphite and diamond are polymorphs, minerals with the same chemistry and different structures.
| Property | Diamond | Graphite |
|---|---|---|
| Bonding | sp3, each atom bonded to four others | sp2, each atom bonded to three others in a sheet |
| Structure | Three-dimensional cubic lattice | Flat hexagonal sheets stacked loosely |
| Mohs hardness | 10 | 1 to 2 |
| Specific gravity | About 3.52 | About 2.2 |
| Appearance | Transparent, adamantine luster | Opaque, gray-black, metallic |
| Electrical behavior | Insulator (except boron-bearing type IIb) | Conducts electricity |
In graphite, the bonds inside each sheet are strong, but the sheets are held together only by weak forces. They slide past each other easily, which is why graphite marks paper and works as a lubricant. Diamond's denser structure gives it a specific gravity of about 3.52, much higher than graphite.
Diamond also conducts heat better than any other natural material at room temperature, which is the property most handheld diamond testers measure. The how to tell if a diamond is real guide explains those testers.
Pressure, temperature and stability
At the Earth's surface, graphite is the stable form of carbon. Diamond is metastable: the laws of thermodynamics favor a slow change to graphite, but the change requires so much energy to break its bonds that at room temperature it does not happen over any human or even geological timescale.
Diamond becomes the stable form only at high pressure. At around 1,000 degrees Celsius, the boundary sits near 4 to 5 gigapascals, which is reached about 130 to 150 km down beneath continents. That is why natural diamonds form deep in the mantle under thick, cold cratons, as explained on the diamond geology page.
Heat changes the picture. In air, diamond burns at roughly 700 to 900 degrees Celsius, turning to carbon dioxide. Jewelers know that a torch can damage a diamond's surface during repairs, which is why they protect stones or remove them before heavy soldering. Without oxygen, diamond survives much higher temperatures before it begins to turn into graphite.
Laboratories reproduce mantle conditions with large presses to grow diamond, or they grow it at low pressure from carbon-rich gas by a different route. The lab-grown diamonds page covers both methods.
Impurities in the lattice
Diamond's tight lattice leaves little room for other atoms, but a few elements can replace carbon. Nitrogen is by far the most common, since its atoms are close in size to carbon. Boron is the other well-known substitute. Hydrogen and, in lab-grown stones, silicon and nickel also appear.
These impurities, along with defects such as vacancies (missing carbon atoms) and distortions of the lattice, cause most color in diamond. Nitrogen absorbs blue light and makes diamonds yellow. Boron absorbs red and makes them blue. The fancy colored diamonds page covers the full range of causes.
Diamond types
Scientists sort diamonds into types based on whether they contain measurable nitrogen and how it is arranged. Labs identify the type with infrared spectroscopy.
Type Ia
Type Ia diamonds contain nitrogen in clusters, and they make up the large majority of natural diamonds, roughly 98 percent. Over millions of years at mantle temperatures, nitrogen atoms migrate and gather into pairs (the A aggregate, IaA) and groups of four around a vacancy (the B aggregate, IaB). A related cluster of three nitrogen atoms, the N3 center, causes the pale yellow of so-called Cape diamonds. Most white and near-white diamonds on the D-to-Z color scale are Type Ia.
Type Ib
Type Ib diamonds contain single, isolated nitrogen atoms. They are rare in nature, well under one percent of natural stones, because nitrogen usually has time to aggregate in the mantle. Isolated nitrogen produces strong yellow to orange-yellow color, the "canary" yellow of many fancy stones. Most diamonds grown by the HPHT method start out as Type Ib, since they grow quickly and have no time for nitrogen to cluster.
Type IIa
Type IIa diamonds have no nitrogen detectable by standard infrared methods. They make up only a small percentage of natural diamonds. Many of the largest and most colorless diamonds in history, including the Cullinan and the Koh-i-Noor, are Type IIa. Some show brown or pink color from plastic deformation of the lattice. Most CVD lab-grown diamonds are also Type IIa.
Type IIb
Type IIb diamonds contain boron and very little nitrogen. They are the rarest type, a tiny fraction of natural diamonds. Boron makes them blue to gray, and it also makes them semiconductors, so they conduct electricity. The Hope diamond, in the Smithsonian, is the best-known Type IIb stone.
Why diamond type matters
Diamond type helps labs separate natural from lab-grown stones and spot treatments. HPHT processing can remove brown from Type IIa diamonds and make them colorless, or turn some Type Ia stones yellow or green. Knowing the type tells a gemologist which treatments are possible and which tests to run. The diamond treatments page covers those processes.
Frequently asked questions
What is diamond made of?
Diamond is made of carbon atoms, each bonded to four others in a cubic lattice. Trace nitrogen, boron or hydrogen may also be present.
Can a diamond turn into graphite?
At surface conditions the change is so slow that it never happens on any human timescale. At very high temperatures without oxygen, diamond can turn to graphite, and in air it burns first.
What is a Type IIa diamond?
It is a diamond with no measurable nitrogen. Type IIa stones are rare in nature and include many large, very colorless diamonds.
Why are some diamonds blue?
Blue natural diamonds usually contain boron, which absorbs red light. They are Type IIb and conduct electricity.