What is the world's hardest wood?

I recently made some toy blocks out of an old gnarled Osage orange fence post. It sure made my planer work, but it got me to wondering: What is the world's hardest wood?
—Ronald Martin, Worthington, Ohio

Interesting question, Ronald. The hardness of wood is measured using the Janka Hardness Test. This test measures the force required to push a steel ball with a diameter of 0.444" into the wood to a depth of half the ball's diameter. Generally acknowledged as the hardest wood, lignum vitae (Guaiacum sanctum and Guaiacum officinale) measures in at 4,500 pounds-force (lbf) on the Janka scale. That's more than twice as hard as Osage orange (one of the hardest domestic woods) at 2,040 lbf and more than three times harder than red oak at 1,290 lbf.

Lignum vitae, found in Central and South America, has long been used in applications requiring extreme durability and density, such as ships' tackle, carvers mallets, and hand plane soles. Naturally infused with a wax-like resin, lignum vitae polishes to a high sheen, making it a prized wood for turners. And because it is self-lubricating and water-resistant, it was, until recently, the preferred substance for shaft bearings for silent-running submarines. In fact, the highest grades of the wood are still sometimes referred to as "bearing" grade.