chemistry

It's mostly based on Si O2, but there are many different types of glass and the other ingredients in the different types vary widely.

glass used to be considered a liquid. The most recent "thinking" that Im aware of is , any non crystalline solid that goes through a glass transition on achieving a melting temperature and the "sorta" phase transition (even though its really not a true "Phase rule" phase, it shares all the attributes of a relatively smooth transition between its various phses.
Glass is always amorphous (no crystal structure) and will, upon suitable time begin to puddle. That's why stained glass windows in medieval cathedrals are slightly thicker at the bases of vertical sections than at the tops, and have to be reset every few hundred years or so.

Many organic polymers and plastics are technically "glass" because they are amorphous and go through the glass transition on heating

I thought I'd read in the last few years that this is actually a myth. While technically glass can flow, at ambient temperatures changes in cathedral would glass would take longer than the age of the universe to become visible to the human eye. The 'bulging' is a quirk of manufacturing and glaziering practice.

http://www.scientificamerican.com/article/fact-fiction-glass-liquid/

Read that, too.

There is even such a thing as steel glass. I think its supposed to be very hard, but has only been produced in extremely thin strips.

amorphous metals, AKA Metallic glasses and glassy metals.
http://en.wikipedia.org/wiki/Amorphous_metal

RAP

I stand corrected. Theres prolly a whole batch of **** I need to discard. The amorphous part was right though. We always look for "Glass" shards in mineral mixes (rocks) . They always show up as black whereas all phases of Xline quartz (and others) will have identifiable colors at given angles of rotation with polarized light.

Heres some lunar glass and olivine xl in a thin section under a ppl analyzer at rather low magnification.