I’m squinting at a diamond in the palm of my hand. As gems go, it’s nothing special: smaller than a grain of rice and full of impurities, it would fetch a poor price. But for researchers like Nester Korolev, those impurities are invaluable for the information they reveal about geological processes under way deep within Earth – all the more so given that some of them look unlike anything we have encountered before. “I hope that we will discover a new mineral,” he  says.

This particular diamond formed around 600 kilometres…