Is It Possible For A Section Of A Tree To Be Harder Than The Rest?

I was cutting down a hickory tree and ran into a section that was like trying to cut steel and just wondered what would make it so hard ..like tree sap or some kind of inclusion/another type of root growing across the grain/ in other words has anyone ever heard of a tree having a petrified section in a live tree...I dulled/wore out three blades cutting this tree down....