I've been researching heat treating recently and it seems that the heat "lines up" the molecules and allows fractures to travel through the stone more easily. I don't think it necessarily makes the stone weaker, although it seems that way because it becomes more brittle. The stone becomes glass-like.
I'm treating ALL my stone that responds to heat. There are drawbacks, though. I've got to learn how to knap this "new" material and it's frustrating. I'm braking a lot more points during the preform stage. It is, however, easier to notch and easier to work with abo tools like antler and bone flakers.