There are three distinct groups of meteorites, classified according to their composition. Those in the first group are composed of the iron-nickel alloy and are called iron meteorites. The second type are made of stone and are called stony meteorites. Those in the third group are a mixture of stone and metal and are called stony-iron meteorites. A trained observer can identify iron meteorites by sight, by chemical testing, or by etching with acid, which causes a characteristic pattern of crisscross bands.
Stony meteorites are the ones most commonly observed falling to Earth.
They are far more difficult to identify by sight, however, because to the untrained eye they resemble other terrestrial stones. Nevertheless, chemical tests and X-ray examinations can positively identify a true stony meteorite. An entirely distinct class of small, natural glassy objects called tektites is sometimes included in the category of meteorites, though these objects are not believed to have come from outer space as meteorites do. Tektites resemble some terrestrial glasses that are formed when large meteorites strike the Earth, but they have certain distinctive characteristics that suggest they came not from Earth but from somewhere within the Earth-moon system. Their exact origin, however, is still uncertain.