The Janka hardness test is commonly used to determine whether or not a type of wood is suitable to be used for flooring. The test measures the force that is necessary to embed an 11.28 mm steel ball halfway into the wood. In the United States, the results are stated in pounds-force, while elsewhere they may be in kilograms-force or newtons.
For example, ebony has a Janka hardness of 3220, which is very high, meaning that ebony is a very hard wood. Balsa wood, on the other hand, only has a rating of 100.
When choosing a hardwood flooring, take into account the Janka rating of the wood and think about what you need for your home.