I thought some folks might find this interesting.
This is an edge picture of a piece of tempered glass after it was broken.
That bit of haze is the remainder of the tension layer after the glass has "exploded" into thousands of tiny pieces.
When intact, tempered glass has an interior tension layer and an exterior compression layer.
Basically the glass is heated and as it heats it expands. When the glass has reached the appropriate temperature it is rapidly cooled by blowing air on the top and bottom - it's the cooling portion of the cycle that determines the level of temper (in psi for us American types), or whether the final product will be classed as tempered or heat strengthened.
As the exterior of the glass cools, the interior remains hot. So while the exterior is contracting (compression), interior remains expanded (tension).
The surface compression layer is going to be about 21% of the glass to each side of the center tension layer - which is center 58% of the total thickness of the glass prior to release of the tension.
Anytime the boundary layer between the compression layer and the tension layer is penetrated, for any reason whatsoever, you are going to have catatrophic failure.
You cannot cut or drill tempered glass. Not diamond saw, not waterjet (waterjets are fun to watch with tempered glass), not laser, not underwater, not flaming string, nothing...there is NO special equipment or techniques that allow you to cut or drill tempered glass.
Many people have claimed that they have successfully cut tempered glass; they have not. Anyone who believes that they have cut tempered glass has cut glass that was not tempered no matter if it was labeled tempered of not.
You can do edge work on tempered glass and you can etch tempered glass. Sometimes the glass won't even explode on you. However, don't, its a very bad idea.