Diamond is a material which has been known about and studied for thousands of years. But, surprisingly, no-one has been able to measure its melting point up until now.
The problem is that diamond is unstable at normal pressures and it can release energy by converting itself into graphite, the material in pencil leads. Normally this would take billions of years... unless you heat it up, when it transforms very quickly, so it is impossible to melt diamond at normal pressures without turning it into graphite first. And the only way of producing the necessary sort of static pressure is crushing materials between two diamonds, which would of course melt.
Now a group based in the Laurence Livermore Laboratory in the US have managed to solve the problem by essentially blowing up the diamond. They applied huge shock waves by hitting it with powerful laser beams; these both compress and heat the material, allowing the researchers to study the light given out.
What this showed is that, at pressures of about 1 million atmospheres, diamond will melt at about 9000C, and it probably melts to form a reflective metallic liquid.
This means that there could be metallic molten diamond at the centre of gas giant planets like Neptune, where it could be playing a role in the planet's magnetic field. The research team hope that, by understanding diamonds a bit better, they might be able to use them as targets for laser fusion plants.