Old English glæs, from glass (v.). Middle English also had an adjective glazen, from Old English glæsen. The glass snake (1736, actually a limbless lizard) is so called for the fragility of its tail. The glass slipper in "Cinderella" perhaps is an error by Charles Perrault, translating in 1697, mistaking Old French voir "ermine, fur" for verre "glass." In other versions of the tale it is a fur slipper. The proverb about people in glass houses throwing stones is attested by 1779, but earlier forms go back to 17c.:
Who hath glass-windows of his own must take heed how he throws stones at his house. ... He that hath a body made of glass must not throw stones at another. [John Ray, "Handbook of Proverbs," 1670]
Glass-house is from late 14c. as "glass factory," 1838 as "greenhouse."
Restricted sense of "drinking glass" is from early 13c. and now excludes other glass vessels. Meaning "a glass mirror" is from 14c. Meaning "glass filled with running sand to measure time" is from 1550s; meaning "observing instrument" is from 1610s.