What could have formed the pattern inside this piece of siltstone? I have what i think is a piece of siltstone with an interesting pattern on the face. It looks like a lichen or lightning bolt, as shown in the picture. The interior of the pattern is the same color as the stone itself, while the negative area is black. I was wondering what may have caused this pattern to form.
Interesting. The photo is low quality so I can not say for sure but possible candidate explanations are:


a septarian concretion
a fulgorite
a dendrite
a differential staining based on differences in porosity


To get a better answer please include some close-ups of the central area and some pictures of the other sides of the rock