Tektites are vitreous materials that are found in association with meteorite impact craters. Their exact origins are not fully understood, but the most widely accepted hypothesis is that they are a kind of glass that is formed by the extreme heat and pressure when a meteorite hits the Earth. They vary in size from microscopic to pieces that weigh several pounds, and are structurally amorphous, being mostly composed of silica glass. They also contain trace impurities of various elements including magnesium, sodium, aluminum, potassium, and iron the relative quantities of these being responsible for the final coloration. Most examples are opaque, but they can be anywhere from transparent to translucent.