How often do simulations like this involve randomness? In the real world, fractures like this are deterministic in the sense that every crack is motivated by something, but often factors like tiny imperfections in a material can come into play. In a graphics simulation, it seems like randomness might help achieve a similar look. But maybe it isn't necessary.
@silentQ Fracture simulation that exactly predicts real-world behavior is (for all practical purposes) essentially impossible for the reason you mention: real physical materials have imperfections, and one will generally not know where these imperfections are a priori (unless, perhaps, you take some extremely meticulous CT scan, say). So, as you say, you might incorporate some model of randomness; ideally this model should at least match statistical assumptions about the material or phenomenon being simulated.