Heat treating makes detaching flakes easier from some kinds of stone. Flakes are longer and the flake scar is smoother, more lustrous and may even feel 'soapy'. But the stone also becomes weaker and more brittle.
Rocks can blowup while being heat treated.
This can happen at any time. Usually this isn't dangerous but it can be. Protect your kiln or oven, particularly if it has a glass viewing window, by putting the rocks in a metal container such as a covered roasting pan. When heat treating rock, first you must dry the rock or it will blow up, explode like bombs, as steam is suddenly released. An hour at 200 degrees F. has proved sufficient for preforms made from quartz, agate and chalcedony from the surface of the western Mojave desert.