Obsidian is a natural glass that is volcanic and looks similar to jet. Back in the days of Early man it would have been used as a sharp cutting tool.
The Egyptians loved glass and made some spectacular pieces including this glass head rest found in Tutankhamun tomb.
Many windows were lost during the Dissolution of the Monasteries and the time od Cromwell. But some examples have survived like this window from St Denys, York.
Nowadays if you think of glass you tend to think of Loius Comfort Tiffany or Rene Lalique.
If you go to the Haworth Gallery in Accrington you can see one of the largest Tiffany collections in Europe.