[djf]

Yup. Tektites. IIRC, there was a SciAm article about it years back.

The study of glass is actually a very interesting part of geology. Turns out every volcano produces as part of the ash, it's own very, very specific kind of glass particles.

I forget the fellows name, but he is an expert in just about every ash-producing eruption for the last 5 centuries or so... and it was his work, in part, that identified Toba as the event it was.

[Bernard Marx]

You're right, tektites are not at all the same as volcanic glass (obsidian). Tracing obsidian sources has been very helpful in archaeology/anthropology since it provides lots of information about cultures that used it for arrow and spear points and ceremonial items. Objects made from obsidian that is unique to the Central Valley of Mexico have been found at archaeological sites in the U.S., for instance, proving the extent of trade that occurred.

There's a great deal of fascinating information about obsidian here: OBSIDIAN