If you heat diamond in the open air, it will start to burn at around 700 degrees Celsius (1,292 degrees Fahrenheit), reacting with oxygen to produce carbon dioxide gas. Heating diamond in the absence of oxygen, meanwhile, will see it transform into graphite – a more stable form of carbon – long before turning into liquid. Despite this, scientists found a way to melt diamond. Using super-strong magnetic fields, they fired small plates at 34 kilometres (21 miles) per second towards tiny pieces of the mineral, subjecting them to 10 million times normal atmospheric pressure. The diamonds melted, then re-solidified when the pressure dropped, leading some scientists to suspect there could be seas of liquid diamond on Neptune and Uranus, due to their atmospheres generating intense pressure and containing carbon.

Answered by Rik Sargent