badgie: You may be refering to the Acacia and Carpinus genus'....locust and hornbeam. They are both commonly know in areas as "ironwood".
But for MY money, that title goes without a doubt to OSAGE ORANGE!!!
When I was in the tree care business, I could take a brand new chainsaw chain that would take a 1/4" sawchip out of dead, aged oak, and try to cut Osage Orange with it and all I'd get is burned dust!!! That stuff is HARD! No wonder native Americans used if for bow and arrows!
