I’ve found that diamond drill bits that are ‘hollow core’ in the center, basically drilling a circle into the glass rather than drilling a hole, work best for me. Here’s a link for an excellent company which provides these affordable bits and gives you a world of helpful information. http://www.diamond-drill-bit-and-tool.com/Diamond-Drill/Default.htm?Diamond-Drill-Bit.htm They even give you information on drilling tempered glass, which they do not recommend. I have only drilled 1/2″ glass with a Dremel and an extention hose for the drill bit. The glass sits on a sponge in the water and I pull the bit in and out of the hole, allowing it to cool. But I can see Petra’s point in using glass as a backer, because of the speed of the drill. The bit at the end seems to be the place where you want to drill the slowest and be the most patient.
Hope that helps!