Black stain in white oak heartwood?

[ORclimber]

I removed a 30" white oak and it had a 10" black stain centered in the heartwood. No signs of rot, and the tree appeared healthy. The staining appeared almost deep purple the second day. Any ideas what turned the heartwood that color? The tree came out to make room for a new house.

 

[Ax-man]

It has some metal in it some where, most old trees do when their close to buildings.