Eight things hidden inside the hardest stone

Almost every diamond is a perfect insulator, but a rare few are blue — and those blue stones can carry an electric current. The colour comes from a scattering of boron atoms, sometimes only a few parts per million, which also turn the crystal into a semiconductor like the silicon in a chip. The most famous deep-blue diamond of all, weighing 45.5 carats, is one of these boron-laced crystals.

A diamond is pure carbon, so it burns. Heat one to somewhere between about 690 and 840 degrees Celsius in pure oxygen and it ignites and turns entirely into carbon-dioxide gas, leaving not a speck behind. In 1772 a French chemist proved it by focusing sunlight through a great lens onto a diamond and watching it vanish — early evidence that the hardest of stones is made of the same stuff as soot and charcoal.

Diamond tops the hardness scale at a perfect ten — nothing else can scratch it. But hardness is not toughness. The same orderly carbon lattice that resists scratching also has clean planes of weakness running through it, and a sharp blow struck along one of those planes can cleave a diamond in two. Cutters use this on purpose, splitting a rough stone with one calculated strike before any polishing begins.

Diamonds form more than 150 kilometres down, where the heat and pressure of the deep mantle pack carbon into its densest crystal. Most are ancient — one to over three billion years old. Then a rare, violent kind of magma drags them upward in a matter of hours and freezes around them near the surface. So the greenish rock that holds a diamond is far younger than the gem trapped inside it.

Almost every diamond carries a little nitrogen, which tints it faintly yellow. But around one or two in a hundred have essentially none — a type so chemically pure it is also the most transparent and the most prized. The largest gem-quality diamond ever cut, a 530-carat colourless giant, is one of these nitrogen-free crystals, as clear as frozen water.

Shine ultraviolet light on a tray of diamonds and roughly a quarter to a third of them light up — and about ninety-five percent of those glow a soft blue. The glow comes from the way trace defects in the carbon lattice soak up the invisible light and release it as visible colour. It fades the instant the lamp goes off, a hidden trait most owners never see in ordinary daylight.

Crack open certain meteorites and you find diamonds — not gems, but specks so small each holds only a few thousand carbon atoms. They are stardust, forged around other stars and dying suns before our own Sun existed, which makes them more than 4.6 billion years old. Trillions of these grains drifted through the cloud that became the solar system and survived inside falling rocks.