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.

A popular belief holds that diamonds are coal squeezed under great pressure, and it is almost never true. Coal forms from buried plant matter within the upper few kilometres of the crust, while natural diamonds crystallise well over a hundred kilometres down in the mantle, far below any coal seam. Most diamonds are also older than any coal could be: many are more than a billion years old, whereas land plants, and so coal, appeared only in roughly the last four hundred million years. The carbon in diamonds comes from the mantle itself and, in some cases, from ocean crust carried down into it.

Because a diamond is so tightly bonded and chemically unreactive, it acts as a sealed capsule, carrying tiny grains of other minerals, called inclusions, from where it grew. These specks are among the only direct samples of the deep Earth that scientists can hold. In 2014 researchers described a diamond from Brazil containing a grain of ringwoodite, a mineral that forms only at depths of roughly five hundred kilometres or more, and the grain held water locked into its crystal structure, supporting the idea that a deep layer of the mantle stores a large amount of water in its rocks. A flaw that lowers a gem's price can be a geologist's treasure.

The Mohs scale, devised by the German mineralogist Friedrich Mohs in 1812, is simply a ranking. He chose ten familiar minerals and ordered them by which scratches which, from talc at 1 to diamond at 10, so a mineral that scratches quartz but not topaz sits between 7 and 8. Geologists still use it in the field because all it needs is a set of test pieces, a steel knife or a fingernail. But the steps are not equal. A mineral two places higher is not twice as hard, and the scale says nothing at all about how far apart its rungs are.

Laboratory tests measure hardness more directly by pressing a shaped point into a polished surface under a known load and measuring the dent it leaves: the smaller the dent, the harder the material. The point is usually made of diamond, since little else survives the job. On such measurements the gaps between Mohs numbers turn out to be wildly uneven. Corundum, at 9, is roughly twice as hard as topaz at 8, while diamond, at 10, is about four times as hard as corundum. That single last step is larger than the entire climb from talc to topaz, which is why so few materials can even mark a diamond.

In 1905, at the Premier Mine near Pretoria in South Africa, a mine manager noticed a gleam in the wall of a pit and pulled out a diamond weighing about 3,106 carats, roughly 620 grams. Named the Cullinan after the mine's owner, it remains the largest gem-quality rough diamond ever found. The Transvaal government bought it and presented it to King Edward VII.

The stone was cut in Amsterdam by the firm of Joseph Asscher, who studied it for months before striking it with a cleaving blade along its natural planes of weakness. It was divided into nine major stones and many smaller ones. The two largest, the Great Star of Africa and the Second Star of Africa, are set in the British royal sceptre and the Imperial State Crown.

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.