Ten Fascinating Facts About Diamonds

Diamonds are the most popular gemstone in the world, but most of the science and history behind them is hidden behind marketing slogans. Here are ten facts about diamonds that go beyond the usual 4Cs introduction — useful whether you are about to buy your first diamond or just curious about the stone itself.

In short: Diamonds are pure carbon formed 150-200km underground over 1-3 billion years, are the hardest natural substance (Mohs 10), and have refractive index 2.42 — which is why they sparkle. Roughly 30% of mined diamonds are gem-quality. The largest ever found weighed 3,106 carats (the Cullinan). The UK 2025 average engagement-ring spend is £3,928, with over 60% of buyers now choosing lab-grown stones.

1. Diamonds are the hardest natural mineral on earth

Diamond scores 10 on the Mohs hardness scale — the maximum. Nothing in nature scratches a diamond except another diamond. This is why diamond-cutting wheels use diamond powder embedded in metal; nothing else is hard enough to shape one. Hardness is also what makes diamond ideal for daily wear: a well-set diamond engagement ring lasts a lifetime with only routine cleaning.

2. Most diamonds are around a billion years old

Natural diamonds form roughly 150 kilometres below the earth's surface under intense pressure and temperature. Most diamonds being sold today are between 1 and 3.5 billion years old. They reach the surface through deep-source volcanic eruptions called kimberlite pipes, which last erupted on this planet around 70 million years ago — so every mined diamond is essentially being recovered, not actively produced.