Iron meteorites originate from the heavy iron cores of asteroids. Over long periods of time, the cores cooled slowly, allowing iron crystals to grow. These crystals show up as cross-hatched patterns known as the Widmanstaetten pattern. Irons are classified based on their structural class, texture, and chemical classification. There are three basic types: hexahedrites, octahedrites, and ataxites. Each has a distinctive structure based on the amount of nickel it contains. The structure depends on the ratio of nickel to iron and the cooling rate of the planetary core where it orginated.