Hi LupinFarm! The stuff on my ceiling is structural glass, specifically Carrara Glass, also known as Vitrolite. You can read more about it here: http://www.nps.gov/hps/tps/briefs/brief12.htm
Here are a few more pics of the glass ceilings in my house:
This room has pink grout. It's the only one that we did not regrout, so it looks a little dirty.
Hall bath ceiling:
This room has tan grout, which we kept.
Master bath ceiling:
This one is my favorite ceiling. I like the two-tone effect, which is also behind the sink and toilet. The grout in this room is a blue-green.
A 1928 ad for Vitrolite that I had framed for the powder room.