Tektites are formed by the impact of large meteors on the Earth's surface. These impacts would shoot sand, rock, and other debris way up in the atmosphere. When this debris fell from the sky and came back to the surface, it was superheated and changed during re-entry. Pieces could fall to the Earth hundreds of miles from the impact sight.
The most famous tektite is Moldavite, a rare green variety of tektite.