Brilliant Glass is created by Jewelcrafters is effectively a 20 hour transmute from uncommon gems to rare or occassionally an epic gem. The chance for an epic gem is believed to be 5%-10%.
If you are trading for Brilliant Glass be aware that the jewelcrafter is not forced to give you the glass that was just created. He could substitute another Brilliant Glass that contains a gem he does not want. Opening a Brilliant Glass does not force you to loot it and a greedy jewelcrafter could simply stockpile Brilliant Glass containing less desirable gems.
Patch changes
Patch 2.4.2 (2008-05-13): No longer requires a forge. Now has a chance to drop an epic gem.