Wood density (hardness) is quantified using the Janka test rating.  Developed to determine the suitability of a wood species to be used for flooring by measuring its resistance to denting and wear, the test measures the force required to embed a .444” steel ball ½ way into a sample of the wood.  The force is expressed in pounds of force (lbf).  The rating varies with the direction of the grain being tested.  Thus testing on the surface of a plank perpendicular to the direction of the grain is listed as side hardness and testing the cut surface of a stump would be rated as end hardness.

Using this rating, Australian Buloke wood with a Janka rating of 5060 (lbf) comes in at the hardest tested.

So how do the woods we commonly use for carving compare?  Below are a few comparisons.  A larger list of tested species can be found on line under “Janka test”.

Australian Buloke: 5060 lbf
Brazilian Teak: 3540 lbf
Mesquite:  2345 lbf
Birch: 1470 lbf
White Oak: 1360 lbf
Pine: 1225 lbf
Black American Walnut: 1010 lbf
Butternut: 490 lbf
Basswood:  410 lbf
Balsa: 22 lbf

Cottonwood bark and cork (also a bark) were not listed as tested due to the limitations of testing protocol.

What this tells us is that the wood we commonly use to form our masterpieces is quite soft.  It does not mean that harder woods can not be used for carving only that the process would require more effort or mechanical means of carving.  Finishing harder wood produces a smoother surface and can be buffed to an extremely high gloss.