Are diamonds really forever?

Diamonds have the best publicity team in the mineral kingdom. They are the hardest natural material, they sparkle, and we are told they last forever. So it comes as a shock to learn that, chemically speaking, every diamond on every ring is quietly trying to become pencil lead. It is just in no hurry about it.

The short answer

Diamond is metastable. At the temperature and pressure of everyday life on Earth’s surface, graphite, the soft grey carbon in pencils, is the lower-energy and therefore more stable arrangement of carbon atoms. Diamond survives only because the conversion is blocked by an energy barrier, which makes it unbelievably slow. At normal temperatures a diamond should last millions to billions of years. That is close enough to forever for most jewellery, but it is not actually forever.

The real story

Diamond and graphite are made of exactly the same thing: pure carbon. The only difference is how the atoms are arranged. In diamond, each carbon atom is bonded to four neighbours in a rigid three-dimensional framework, which is why it is so hard. In graphite, each atom bonds to three neighbours in flat sheets, and the sheets are only loosely held together, which is why graphite smears onto paper.

Deep underground, where pressure is enormous, diamond is the winner and is the stable form of carbon. That is where natural diamonds form, in Earth’s mantle. Bring them up to the surface and the rules change: graphite becomes the preferred state. But to rearrange itself, a diamond’s atoms need a big push of energy to break their bonds and resettle. At room temperature they simply do not get that push often enough to make any noticeable difference.

Heat changes things. Without oxygen, at very high temperatures of roughly 1,500 to 1,900 degrees Celsius, diamond can turn into graphite much more quickly. And with oxygen around, there is a more dramatic fate. Because diamond is pure carbon, it can burn, combining with oxygen to form carbon dioxide gas, just like coal does. In 1694, the Italian scholars Giuseppe Averani and Cipriano Targioni focused sunlight through a large lens onto a diamond and watched it vanish. Not shatter. Not melt. Vanish.

Why people get it wrong

We confuse hard with indestructible. Diamond is extremely hard, meaning it resists scratching, but hardness says nothing about chemical stability or toughness. A diamond can still chip if struck the wrong way, and it can burn. On top of that, a century of romantic advertising has drilled the idea of eternal diamonds into popular culture, so the slow chemistry rarely gets a mention.

The good news for anyone wearing one: on any human timescale, nothing is happening. Your grandchildren’s grandchildren will not notice any change. The slow slide to graphite is real, but it is measured on the scale of geology, not family heirlooms.

“So the fanciest rock on the planet is basically a pencil that hasn’t got round to it yet. Relatable, mate. I’ve got a few jobs like that myself.”