At least they're not long for this world in the incandescent interiors of giant planets - or even medium sized ones . Though diamond's melting point rises all the way up to over twice the pressure at the center of the Earth , it nosedives shortly thereafter .
So, suggests a new paper in Nature Materials you should abandon all hope if you drop the Hope Diamond overboard on Neptune --It's going to melt before it hits bottom, and encounter something even weirder as it sinks:
Because several of the high pressure polymorphs of solid water melt at temperatures higher than its Earthly boiling point , the forlorn Hope may encounter a lot of hot ice on the way down.
Laser-shock compression of diamond and evidence of a negative-slope melting curve Stéphanie Brygoo , Emeric Henry et al.
"In spite of its fundamental and planetary importance, the stability domain of this strong covalent material is largely unknown. After decades of experimental efforts, ... Accurate Hugoniot data of diamond exist up to 590 GPa. Higher-pressure measurements along the diamond Hugoniot have recently been achieved by laser shocks, showing that diamond probably melts to a conducting fluid.
We report here laser-shock Hugoniot data across the melting transition. The shocked diamond crystal begins to melt around 750 GPa. Furthermore, a negative volume discontinuity at melting is observed. This requires a negative melting slope and thus supports the existence of a maximum on the diamond melting curve...the stability domain of the diamond crystal is now constrained in a relevant region for Uranus-like planetary interiors.