Tektites are another form of naturally occuring glass.
The consensus these days is that they are cooled droplets of the earth's crust flung up during an asteroid impact - or during large volcanic events.
Thailand is well known for dark green / black tektites; in Australia we have black (smoky when polished) ones (called Australites - got a nice one of these from Kalgoorlie); but the prettiest of all are Moldovites - champagne bottle green and often intricately etched.
Just to throw in a word link - there's a whole terminology associated with tektites. Things such as buttons, flanged buttons, teardrops, partial flanges, dumbells - let alone the various types of tektites. Have a squiz at the URL given...
Good URL at
http://www.crystal-world.com/html%20pages/meteorite%20pages/tektites/glassy_tektite.htm It mentions several other types as well.
BTW - all glass is, in geological terms, quite unstable. Glassy rocks and tektites relatively rapidly break down to clay.
stales
