It was a minor plot point in Arthur C. Clarke's Space Odyssey novels that the core of Jupiter was essentially an Earth-sized diamond, something to do with all those hydrocarbons being blasted by lightning into carbon soot, which fell towards the core, where the temperature and pressure transformed it into diamond. [spoiler]Later on in the series, enough of that diamond is liberated and becomes the downfall of all the diamond cartels back on Earth.[/spoiler]