Stones constitute the largest group of meteorites. They originated in the outer crust of a planet or asteroid. Recently-fallen stone meteorites are covered by a thin, black rind known as fusion crust, which forms as the rock’s surface is burned during flight. Fusion crust is fragile and deteriorates easily, so stone meteorites that have been on the surface of our planet for a long time have a similar appearance to Earth rocks. Visible inside most stone meteorites are tiny, glassy, spheres known