Lignum Vitae is considered the hardest commercial wood. With a hardness of 4500 it is roughly three and a half times as hard as red oak. This wood is not only extremely hard, but it is also extremely heavy, with a specific gravity over 1.0 it will sink in water.
The hardness of lignum vitae makes it ideal for situations where durability is necessary. Historically it has been used for making things such as pulleys and propeller bushings for boats. It also excels where high density is valued such as in a woodworking mallet.