fisholith

You could draw white cracks on a black texture and then overlay (multiply) a white to black radial gradient over it to make it fade from full brightness in the center of the texture to black at the edges. Then drag a brightness threshold over it from light to dark, and it will look like the cracks are spreading out. A little refining and it should look pretty good.

See the image bellow for a simple example…


Take care, and good luck!