I was reminded of a bead I used to own, that under a microscope appeared to be some sort of powder glass.
Here's the discussion from 2008 (!)
http://beadcollector.net/cgi-bin/anyboard.cgi?fvp=/openforum/&cmd=get&cG=5363831393&zu=3536383139&v=2&gV=0&p=