As reported by Bloomberg Quint, one of the biggest diamonds in history – the fifth biggest, as it so happens – has been found, this time from within the Letšeng mine in the kingdom of Lesotho, a small landlocked nation in southern Africa.
Diamonds are usually measured in weights, with one “carat” equal to 200 milligrams – to you and I, a nearly imperceptible weight. This new diamond weighs 910 carats, equivalent to 182 grams (6.4 ounces), which is roughly the same as a hamster, two standard decks of cards, or three-fifths as heavy as the average adult human heart.
The biggest diamond ever found was the 3,106-carat (621-gram/1.37-pound) Cullinan, found in South Africa in 1905.
The official report from Gem Diamonds – which owns 70 percent of the Letšeng mine – notes that it’s a D color Type IIa diamond, which means that it’s incredibly pure, containing few nitrogen atoms. This means that when it’s sold, it will likely fetch one of the highest prices for a diamond in the world.
Bloomberg notes that another diamond from the mine, 2006’s 603-carat uncut “Lesotho Promise”, sold for $12.4 million. In fact, the mine is the highest dollar value per carat diamond mine in the world.
Like plenty of diamond mines, Letšeng is volcanic in origin. The mine is digging down into a kimberlite, a strange volcanic feature part of a wider class of volcanoes named maar-diatremes.
Kimberlites are especially weird. Generally only appearing in the oldest continental crust on the planet, the magma contained within these pipes was once incredibly hot, incredibly fluid, and very gassy.
As pointed out by Wired, the CO2-rich foam ahead of the magma is so buoyant that it ends up moving at speeds of 1,000 kilometers (621 miles) per hour, faster than a passenger jet. The magma within them originates from deep below the crust, within the otherwise solid, superheated mantle.
These ancient eruptions don’t happen anymore, and it’s likely that their violence and deep-rooted source magma was a product of a once much hotter mantle.
Diamonds formed deep beneath the crust too, 1 to 3 billion years ago – not from coal, by the way, but from highly compressed, profoundly heated carbon-rich deposits in narrow stability regions within the upper mantle.
Kimberlite eruptions happen to bring up chunks of the upper mantle with them to the surface, and within these alien rocks – these xenoliths – you can get diamonds. Fascinatingly, we know the eruption process has to happen in a matter of hours; a slower eruption would allow them to gradually rearrange their chemical structure into graphite.
Letšeng's success is partly down to the fact that it sits on a series of kimberlite pipes, which reside above a part of the upper mantle once perfectly suited for growing diamonds.