Epoxy-based glue and super glue both work when gluing glass to another piece of glass. These are ideal because of their strong hold and because they dry clear.
The benefit of using super glue over epoxy glue is that it is easier to use. It is also good to have around since it can also be used for other craft projects at home. Epoxy glue provides a few extra steps, but it also gives a stronger hold. In either case, be careful when applying the glue. Don't let fingerprints or sweat on the glass or glued area, as it can't be removed from the adhesive. The glass surfaces must be clean and dry before attempting to glue them together.