The standard by which hardwoods are tested and rated is called the Janka Hardness Test. This test measures the pounds per square inch required to embed a 0.444 inch steel ball to half its diameter in the wood. It is an indicator of the ability of a wood species to withstand denting.
Note that the scale is linear. So a wood with a hardness of 1800 is twice as hard as a wood with a hardness of 900.