Tektites from all over the world
Tektites are pieces of natural glass, of sizes ranking from specks barely visible to the naked eye up to blocks of a football; they are usually black, but sometimes green or yellow. They have been treasured by those who found them, from modern aerodynamicists back to Stone Age hunters of the mastodon and the mammoth; interest in tektites is thousand of years older than interest in gold or silver. (John A. O´Keefe: 1976 in his book "Tektites and their origin").
