Iridium is a very hard, heavy, rare and expensive silvery metal and the most noble of all metals. It hardly reacts, but as a fine powder it is flammable. It is used is special alloys and as a catalyst. Crucibles for high temperature applications are made of iridium. The meteorite, which 65 million years ago played a key role in the extinction of the dinosaurs, contained relatively much iridium. This can still be observed in sediment layers from this time.