Most of these are stones or metallic masses of only a few kilograms, small enough to be held in your hand. The rarer, larger falls are produced when a mass of hundreds or thousands of kilograms strikes the atmosphere, often breaking up to scatter fragments over a large area. Even larger projectiles can also strike at intervals of thousands of years, producing impact craters when they crash into the surface. A surprising variety of rocks are in the Earth's meteorite collections. The traditional description of these meteorites are based on their appearance and bulk composition.
Iron meteorites are nearly pure metallic nickel-iron.
Their extraterrestrial origin is obvious when we bear in mind that iron and most other metals naturally occur on the earth in the form of oxides rather than in the pure metallic state. The stony meteorites more closely resemble terrestrial rocks, and they are not generally recognized as extraterrestrial in origin unless their fall was witnessed. The stony-irons contain a mixture of stone and metallic iron, as suggested by their name. These are known as primitive meteorites . All primitive meteorites are stony, although not all stony meteorites are primitive.