Movie star Marilyn Monroe once famously sang, "Diamonds are a girl's best friend". Scientists in Belgium believe they have unearthed an 18-km wide layer of these precious stones, on Mercury. Our nearest planetary neighbour could be a celestial jewel. Researchers tested how Mercury formed, 4.5 billion years ago. Mercury evolved from a spinning cloud of dust and gas. Over millions of years, the dust was compressed into graphite. It is unlikely that Mercury's diamonds could ever be mined. This is because they are 500 km below the planet's surface.

Researchers used a special machine to simulate the conditions under which Mercury formed. The machine is used to make synthetic diamonds. Researchers made a mixture of silicon, magnesium and aluminium inside a graphite capsule. This was subjected to pressure 70,000 times greater than Earth's. It was heated to temperatures of 2,000ºC. A researcher speculated about the diamonds on Mercury. He said: "They would [resemble] pure diamonds." Scientists believe there are a quadrillion tons of diamonds beneath the Earth.