The Janka rating is determined by the number of pounds of pressure required, to press a stainless steel ball, with a diameter of .444 inches, halfway into the side of a board. Do not take hardness to mean it's the best one to select.

MaterialRelative Hardness
Douglas Fir660
Yellow Pine690
Southern Yellow Longleaf Pine870
American Cherry950
Black Walnut1010
Heart Pine1225
Birch1260
Red Oak1290
Beech1300
Ash1320
White Oak1360
Maple1450
Bamboo (average)1820 (varies)
Hickory/Pecan1820
Purple Heart1860
Santos Mahogany2200
Brazilian Cherry2350