Brittle Fractures:
Brittle materials experience little or no plastic deformation and a very low energy absorption rate before and while fracturing. This means that the fractures can initiate very easily at points of high stress concentration and that brittle materials experience spontaneous crack propagation, whos' speed is almost instantaneous. These two factors mean that brittle materials can fracture very easily and so are not considered very tough.
The material, once a crack has begun can be considered to be unstable. Meaning that once started the fracture requires no extra force in order to propagate, able to continue due to the force that it exerts on the material in front of it.
The cracks and fractures in the material propagate perpendicular to the applied stress and due to the surface morphology of amorphous materials, the fracture surface comes out flat and hard.
This also means that there is no obvious order or pattern to all of the brittle fracturing.