The Science of Gemstones

Rough diamond – the giant Cullinan stone as found in 1905

Fortunately for diamond-lovers and geologists alike, some of these riches have been blasted to the surface, in unique kind of eruption that has its roots far deeper than any conventional volcano.

Through cracks in the deep crust, lava has gradually seeped upwards, carrying diamonds as passengers. This process should spell doom for the diamonds. The pressure is dropping all the way, and by the time it reaches the surface, reactions within the hot lava should turn the diamonds into graphite, the form of carbon that’s stable at low pressure.

Diamonds can only survive if they are spewed out in the surface suddenly, so that they “freeze out” in their unstable form. And, by a miracle of Nature, that’s exactly what has happened in southern Africa and Australia.

Here, the diamond-rich lava is also full of carbon dioxide gas. Once it’s within two miles of Earth’s surface, the pressure of gas is higher than the weight of overlying rock. Exactly like a shaken bottle of champagne, the carbon dioxide erupts upwards. It punches a narrow “pipe” up through the rocks, and bursts through onto the surface, spraying out rocks and diamonds.

Today, we find these pipes filled with bluish kimberlite rock, with diamonds scattered throughout it. The diamond mines of South Africa – and now Australia – provide an invaluable commodity not only to jewellers and industry, but also to geologists. The diamonds give us a unique insight far into the Earth, not just in the form of the gem itself, but because some diamonds carry tiny fragments of unusual rocks with them, our only samples of the planet from so deep beneath our feet.

The 621 gramme Cullinan diamond was divided into 11 pieces

Rubies and sapphires

Throughout history, rubies and sapphires have had their own separate folklore and attributes. But, astoundingly, they are actually the same gem – and identical to black crystals of emery, used on an emery board for filing your nails. The only difference lies in slight traces of impurities.

Corundum is the name that covers all these minerals. It’s a tightly bound array of aluminium and oxygen atoms. These two reactive elements bind together with immensely strong chemical bonds, making corundum the second hardest substance after diamond. In its most impure and cheap forms, such as emery, it’s a vital abrasive for industry.