Which of these is the hardest natural wood known to man?

Wood, as a material, can have its hardness quantified using the Janka rating; the industry standard rating. With this rating, the resistance of wood samples to denting and wear is measured. Varying on account of wood grain direction, different woods are tested based on the cut surface of a stump cut from the material. The measure of hardness and strength is pounds-force (lbf). Australian Buloke is an ironwood tree that is native to Australia, this wood comes from a species of tree occurring across most of Eastern and Southern Australia. Known as the hardest wood in the world, this particular type has a Janka hardness of 5,060 lbf.

Source: Hitchcock and King