Basically: get position in texture...voronoi diff scalar... constant scalar for initial size....color adjust with white and black reversed and narrowed to 0.3 and 0, and gamma lowered to maybe 0.3 or so, then maybe some warp, and you have cracks. For smaller cracks, use a transform shader. If that's what you mean.
If you mean cracking the sides of a voronoi, you may think of breaking up in a surface shader by smaller cracks.