Stresses in that stuff can do strange things. (Didn't someone here post a link to a Prince Rupert's drops clip a short while ago?)

My preference would be to do it in one pass. Even with a drill press, it may be hard to line up the hole exactly because the shape of the glass makes it so hard to secure. The size of the hole is small enough

If you go diamond bit, you will need a continuous water stream or you'll burn up the bit.

Some more info:
http://www.diamond-d...Glass-Stone.shtml