Tektites

The prevailing theory for quite a while was that tektites were the result of a huge meteoroid or asteroid collision with the moon. According to this theory, molten debris from the collision was thrown so forcefully off the Moon that it eventually solidified and was drawn by gravity down to the Earth's surface.

More recently, evidence has emerged indicating that tektites are the product of meteoritic impacts right here on Earth. Scientists speculate that the sheer force of the impact created a molten mix of sand and other materials which splashed high up into the earth's atmosphere, solidified, and rained down again in the four parts of the world where tektites are found.
 
Several natural glass types are found on the Earth. Tektites are one of these; at a glance they sometimes resemble obsidian, but under a microscope tektites look more like glass than obsidian because they have almost no mineral crystals in their composition. Tektites also have much less water in their composition than obsidian.

There are several areas in which tektites can be found, but the most famous are Indonesia and the Moldau region of Europe.