Any of numerous dark-brown to green glassy objects, usually small (about the size of a walnut) and round with pitted surfaces. Tektites consist primarily (65% to 90%) of silica and have a very low water content. They are found in groups in several widely separated parts of the world and bear no relation to surrounding geologic formations. Some have shapes that show the kind of melting and deformation typical of objects that fall through the Earth's atmosphere. Tektites are believed to be of extraterrestrial origin or to have formed during high-velocity impacts on terrestrial rocks.