Different species of wood have different densities and therefor, some are harder than others. The Janka scale is a scientific method to measure that hardness and give some insight to how durable a particular species will be.
The Janka hardness test measures the hardness of wood. It measures the force required to embed an 11.28 mm (0.444 in) steel ball into wood to half the ball’s diameter. This method leaves an indentation. It is a good measurement technique to determine the ability of a hardwood flooring can withstand denting and wear. It is also a good indicator of how resistant it is to sawing and nailing.