Why Are Diamonds So Hard? Carbon in Two Forms

A carbon atom has four electrons available for bonding, and in diamond it uses all four to form single covalent bonds to four neighbours arranged at the corners of a tetrahedron, each of which does the same, so that the whole crystal is one continuous molecule. Every bond is short, strong and pointed in a fixed direction, and there are no weak links anywhere; to scratch a diamond is to break carbon-carbon bonds, and to break it is to break a plane of them at once. That is why diamond sits at the top of the Mohs scale of scratch hardness, at 10, with corundum, the material of sapphires, a distant second, and why it has the highest thermal conductivity of any natural material, five times that of copper, since vibrations pass along the stiff lattice almost without loss. It is not indestructible. It is brittle, and a hammer blow along the right plane will cleave it, which is how cutters shape it.

In graphite each carbon bonds to only three neighbours, in flat hexagonal sheets like chicken wire, and the fourth electron is shared across the whole sheet in a delocalised cloud. Within a sheet the bonds are even stronger than diamond's, which is why a single sheet of graphite, graphene, is the strongest material ever measured. Between the sheets there are no bonds at all, only the feeble attraction that holds any molecules together, so the sheets slide over each other at a touch. Rubbing a pencil across paper shears off layers and leaves them behind, and the same sliding makes graphite a lubricant and the delocalised electrons make it conduct electricity, which diamond does not. The other forms of carbon:

• •Diamond: every atom bonded to four, a rigid three-dimensional lattice, hardest, transparent, insulating
• •Graphite: every atom bonded to three in sheets, soft, black, conducting
• •Graphene: one isolated sheet of graphite, first peeled off with sticky tape in Manchester in 2004
• •Fullerenes and nanotubes: sheets curled into balls of 60 atoms or into tubes, discovered in 1985 and 1991
• •Amorphous carbon: soot, charcoal and coal, with no long-range order at all

Diamond is the stable form of carbon only under pressure. At the Earth's surface graphite is the stable form, and a diamond is, strictly, slowly turning into graphite; the process is so slow at room temperature that it will take longer than the age of the universe, which is what forever means in the advertising. Natural diamonds form 150 to 200 kilometres down in the mantle, where the pressure is 50,000 times atmospheric and the temperature above 1,000 degrees, and they reach the surface only in the rare volcanic eruptions of a rock called kimberlite that shoot up from that depth fast enough not to cook the diamonds back to graphite on the way. Most are between one and three and a half billion years old. A few come from elsewhere: tiny diamonds form in meteorite impacts, and some meteorites carry diamonds that condensed in the atmospheres of dying stars before the Sun existed.

General Electric made the first synthetic diamonds in 1954 by squeezing graphite in a press at 55,000 atmospheres and 1,400 degrees with a metal catalyst, and that method still makes most of the diamond grit used for cutting, grinding and drilling, which is by tonnage the diamond industry's main product. Since the 1990s a second method has grown gem-quality stones by chemical vapour deposition, breaking methane into carbon atoms in a plasma so that they settle layer by layer onto a seed crystal, and laboratory diamonds, chemically identical to mined ones and distinguishable only by specialised instruments, now make up a large and rising share of the jewellery market at a fraction of the price. Diamond is also becoming an engineering material: as heat spreaders for electronics, as windows for lasers, and, because a defect in the lattice can hold a single electron's spin, as a candidate for quantum sensors and computers.

Diamond is hard because each carbon atom is bonded to four others in a rigid, continuous three-dimensional lattice with no weak planes, while graphite, the same element, bonds each atom to three in sheets that slide freely, which makes it soft, black and conducting. Diamond is stable only under the pressure of the deep mantle, where natural stones formed billions of years ago before kimberlite eruptions carried them up, and it has been made in presses since 1954 and grown from gas since the 1990s.