Not so interesting...

I disagree. Looks almost like someone put marker on the inside

And what is it?

I would think its scarring from someone carving something into it.

Almost looks like Ogham.

Im trying to decipher this, looks like Ogham writing

From reddit user StrykerSeven: It is a rather nice cross section of the nesting behaviour of wood-boring insects. The mama chews her way into the secondary growth of the tree in the late summer/fall and at intervals she will veer off and chew out 'egg galleries' which will host egg clutches until spring. She keeps the entrance to the gallery packed with frass and poop to make a plug that keeps out cold and lets in a bit of oxygen. 
The wood staining is a result of one of the fungal species that the beetles carry, which mature in the galleries in the spring and can totally cut off nutrients to the tree, or simply weaken it, or do nothing much at all, depending on the species of fungus and tree. 
Edit If you zoom in you can actually see the beetles still in the galleries that haven't emerged yet. Looks like maybe Emerald Ash Borers.