Diamond

Diamond is the hardest naturally occurring substance on Earth, a Mohs hardness of 10 (a defining endpoint of the scale), composed entirely of pure carbon in a tightly bonded crystal lattice. The same carbon atoms that compose soft graphite (Mohs 1) arrange in diamond into a 3D tetrahedral network that resists virtually all natural mechanical wear.

Diamonds form 100-150 miles below Earth's surface in the upper mantle, at temperatures of 1,800-2,000°F and pressures of 45-60 kilobars. Most natural diamonds are 1-3 billion years old. They reach the surface through rare violent volcanic events — kimberlite pipes — which explode upward from deep in the mantle at supersonic speed, carrying diamonds suspended in the rising magma. Pipe eruptions are too fast for the diamonds to convert to graphite during the trip; once cooled at the surface, the diamonds are metastable but persist essentially forever at surface temperatures.

Major diamond-producing regions include South Africa, Botswana, Russia, Canada, and Australia. The traditional Belgian and Israeli cutting centers, plus growing operations in India, dominate the cut-diamond trade. The "Big Hole" open-pit mine at Kimberley, South Africa — the original kimberlite locality — was hand-dug starting in 1871 and is the largest hand-excavated hole on Earth.

The De Beers diamond cartel controlled most of the global rough-diamond market through the 20th century, deliberately holding back supply to keep prices high and famously creating the "diamonds are forever" engagement-ring tradition through a 1947 advertising campaign that remains one of the most successful in marketing history.

Synthetic (lab-grown) diamonds are now indistinguishable from natural by everything but specialized lab equipment, and they are dramatically cheaper. The synthetic diamond industry has grown rapidly through the 2020s with significant impact on natural diamond markets. Industrial diamonds (used in cutting, grinding, and drilling tools) are almost entirely synthetic.

Diamond is the April birthstone. The Hope Diamond, Cullinan Diamond (cut into the Star of Africa in the British Crown Jewels), and Koh-i-Noor are the most famous individual diamonds in history.