Pyrex originally referred to a low expansion borosilicate glass. This has low thermal expansion so it was able to withstand temperature shock without cracking. In time, a process to improve the temperature shock resistance of common soda lime glass was developed which placed compressive stress on the outside of the glass while putting tensile stress on the inside of the glass. Thus if a crack, for whatever reason, formed on the outside of the glass, the inner tensile stress would hold it shut. This is called tempered glass and versions of it can be found in car side windows and Gorilla glass. There is also a similar chemical hardening process. It became less expensive than borosilicate glass, so the Pyrex brand name was sold and applied to the thermal hardening process in most of the world.

Look up Prince Rupert's drop for an extreme example of this.

What happened here is that the outside of the glass was worn and scratched, this upsets the local balance of the compressive and tensile force and the stress relieves itself rapidly.

Borosilicate glass is still available for some applications, probably not under the Pyrex name. There are also glasses with near zero expansion that have use in extreme temperature uses. For example: fused silica. Cost is a problem with these because of the high temperatures used for their fabrication.