In alpha 7, youtube videos have shown that if you keep adding silicates to a planet like earth, it'll become a gas giant and then become a star. So you have a pure silicate star. But we don't know what would actually happen if you had a huge mass of metallic elements in one place... it might not start fusion, since fusion always starts with hydrogen. Would it be a REALLY huge rock, like a giant planet? Maybe it would be like a giant brown dwarf, only going through fusion really slowly.
This is why US2 will be a tough game to make. You'll have to account for theoretical things that can't happen in real life. That's why they hired scientists to help with the game.