The Science of Gemstones

Diamonds, rubies and sapphires captivate us by their symbolic values of wealth and power, and also by their heavenly beauty. But these gems are born in hellish fires deep below Earth’s surface.

Look at the diamond in your ring, and it’s not only the most valuable stone you’re likely to ever handle. It’s also the oldest; it’s come from deeper in the Earth than any other; and it’s the rock that has literally been through Hell…

Diamonds can’t form on the surface of the Earth. The ultimate gemstone is actually unstable at the pressures we live in. Diamond is made from pure carbon, and the diamond in your ring is trying to change to graphite, the form of carbon that’s most stable in our environment – and best known as black pencil “lead”. Fortunately, the transition takes place at an infinitesimally slow pace.

Beauty and the beast: diamonds were born from tortured rocks 100 miles down

There’s only one place that diamonds can be forged – deep beneath our feet, at least a hundred miles down. That’s as far below us as the Space Shuttle orbits above us; and far deeper than any borehole ever drilled. Down here, we are truly in Hell: the pressure is 100,000 times higher than the pressure of air around us, and the temperature is around 1000C.

The story of diamonds begins three billion years ago, when the Earth was still young. After its fiery birth, the surface of our planet solidified – not into a single uniform piece of crust, but into several continental blocks. Each block was shaped like a ship, with a solid central “keel” reaching a hundred miles deep into the hot viscous rocks of the Earth’s mantle.

Within the keel, there was a unique environment. It was relatively cool and stable, amid the hot and ever-mobile mantle rocks. Water, carbon dioxide and methane were dissolved at high pressure in the mantle. They seeped into the keel, and reacted together. The carbon precipitated out. Under the intense pressure down here, the most stable form of carbon was diamond.

Nowhere else on Earth – nor at any other time – could diamond form so readily. After this period, continents never again formed such deep keels: instead, these rocky ships started to drift around the globe in the process of plate tectonics. Today, the bottoms of the oldest continents still carry their ancient load of gemstones. The roots of Australia, Canada and southern Africa are studded with a richness of diamonds that surpasses imagination.