Tempered glass is glass subjected to a process that permanently changes its structure, making it tough and durable. Any cutting or drilling necessary is carried out before the tempering process. It is inadvisable to drill tempered glass.
Tempered glass has a very hard, durable surface. It is more resistant to heat than standard glass and breaks less easily. Pieces of glass that are to be tempered are precut and drilled before undergoing the tempering process, because they cannot be drilled afterwards.
Tempered glass is made by heating glass to around 1300ºF (700ºC). The glass is then removed from the furnace and cooled very rapidly. This creates new stress patterns in the glass; the core of the glass is under tension, while the surfaces of the glass are in compression. Tempered glass is 400 to 500 per cent stronger than standard glass, but if the surface is damaged the whole pane shatters completely. This applies to cutting and drilling as well as accidental damage.
Aside from the high likelihood of tempered glass shattering if you attempt to drill it, the hard surface of tempered glass significantly reduces the life of diamond drill bits.