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.