Marilyn Monroe once sang, "Diamonds are a girl's best friend". Scientists believe they have found an 18-km wide layer of these precious stones, on Mercury. Earth's closest neighbour could be a huge jewel. Researchers tested how Mercury formed from spinning clouds of dust. Over 4.5 billion years ago, the dust became graphite. Mercury's diamonds may never be mined. This is because they are 500 km below Mercury's surface.

Researchers used a machine to simulate the conditions under which Mercury formed. They mixed silicon, magnesium and aluminium inside a graphite capsule. This was exposed to pressure 70,000 times greater than Earth's. It was heated to temperatures of 2,000ºC. A researcher the diamonds on Mercury would resemble pure diamonds. Scientists believe there are a quadrillion tons of diamonds beneath the Earth.