Liquid Diamond Seas — It’s very, very, very hard to melt a diamond. At 1292 degrees Farenheit, they burn if oxygen is present. If oxygen is absent they simply turn into graphite.
It is possible to melt them but requires 10 million time normal atmospheric pressure.
Scientists think Neputune and Uranaus have this pressure along with large quantities of carbon. They say it is possible that these planets have seas of liquid diamonds.