Gluing glass can be tricky. Because glass is so smooth and non-porous,it won't bond with many adhesives. Glues that seem to work well initially can become brittle, lose their adhesion, or become discoloured. There are glues that work well with glass, however. The choice of adhesive depends on your project -- a glue that works well if you are gluing paper to glass, for example, may be inappropriate if you're gluing two glass elements together.
GS Hypo Cement
Also known as watch crystal cement, this adhesive comes in a specially designed tube with a long, fine nozzle, like a hypodermic syringe. This allows you to apply the adhesive accurately. The adhesive itself is formulated to bond with many materials including glass, plastics and metals. It stays wet for longer than some other adhesives, allowing you more time to make adjustments. Watch crystal cement dries water-clear, meaning that it's completely transparent and colourless. This makes it suitable for jewellery and for other applications where the colour and general appearance are important.
E6000 is a widely used jeweller's glue that is 1 part epoxy glue. It requires no mixing. You can use E6000 to bond glass with a variety of non-porous materials including metal and vinyl, as well as with porous materials such as wood and fabrics. E6000 will not bond glass to glass, however. E6000 is touch-dry in around 15 minutes, and cures fully in 24 hours. It dries clear and flexible.
Beacon 527 Glue
527 glue is similar to E6000, but has a thinner, less viscous consistency. The nozzle of the tube comes to a finer point. These two features make it easier to apply very small amounts of 527 glue.
Aquarium glue is the name given to a class of watertight adhesives designed for use with aquaria. Generally, silicone-based, aquarium glues have the advantages of being clear, strong and inert when dry. Unlike E6000, you can glue glass-to-glass with aquarium glues.
Silicone Gel Adhesives
Silicone gel adhesives are often used as découpage media. You can use them to glue paper to glass, for example when gluing photos or patterned paper to the underside of a flat-backed marble or tile. Silicon gel glues dry clear and are available in acid-free varieties, which won't discolour paper over time. You can use an extra coat of silicone gel glue as a sealer to stop paper or other materials from peeling away from the glass.