A meteorite is any fragment of rock from the solar system that passes through the atmosphere and survives its impact with Earth. Silicate-based minerals such as olivine and pyroxene make up most meteorites - up to 95 percent of known finds - so scientists call them "stony." The vast majority of stony meteorites, dubbed chondrites, contain small, rounded inclusions called chondrules. Scientists believe these chondrules are bits of formerly molten dust that once swirled in a giant disk around the infant Sun during the formation of our solar system 4.6 billion years ago. Chondrules may be the oldest and most primitive materials in the entire solar system. A few stony meteorites have been partially or mostly melted and recrystallized since their formation; these are called achondrites, since they do not contain chondrules. Some achondrites may be pieces of asteroids, or even the Moon, that broke off their parent body due to a large impact.

The mineral composition of meteorites ranges from nearly solid iron and other metals to nearly solid silicates. In general, iron meteorites are much denser than stony meteorites. If two meteorites of the same size enter Earth's atmosphere, the stony meteorite is more likely to break into pieces and burn up completely than the iron meteorite. In fact, some carbonaceous chondrites, which contain organic compounds and water in addition to silicates, can crumble in a human's hand. Other stony meteorites are hard to the touch and may resemble terrestrial rocks. They contain silicate mineral grains, with some metal grains mixed in. These stones came from smaller rocks and dust that accreted (stuck to each other) during the earliest days of the solar system, 4.5 billion years ago.

Chondrules, small spherical inclusions inside a stony meteorite, are usually less than 1 cm (0.4 inch) in size, but they provide huge clues to the formation of our solar system. The silicate minerals olivine and pyroxene are the most common ingredients of chondrules, though their outer layers may consist of feldspars or other silicates. Scientists think that chondrules started out as aggregates, or clumps, of small, primitive dust particles, which were quickly heated by some cosmic event - perhaps some outburst from the young Sun, or simply the friction from small bodies ("planetesimals") crashing into each other. Some chondrules contain even smaller grains of silicates that may be some of the oldest materials in the solar system. Scientists measure the amounts of trace radioactive isotopes in chondrules and meteorites in order to estimate their ages.