I know I just posted something else, but this deserves it's own thread(I hope @Catelite hasn't beat me to it)...
Anyway, this is ice which, when melted, turns into Diamond. If put under pressure, the ice becomes snow, which can be melted into diamond. This can be done by creating a block of ice or snow and typing(without quotes) "set ctype elem dmnd". Replace elem with either ice or snow, depending on what you placed.