Petrified Wood with black layer?

[diabeticwolf]

This piece of petrified wood that I collected from the river rock in my grandma's backyard where she used to have a man-made pond has a black layer that goes all the way around, and I was wondering if anyone could possibly tell me why it may have this layer?

[Rockwood]

Perhaps the wood dried enough to crack from shrinkage before it was deposited in the spot that it petrified. Trees often have imperfections in their wood anyway. In cold environments it's common for them to essentially graft themselves back together in the spring.