One inaccurate test for the hardness is to take a hard object (a knife will do) and push it with moderate pressure into the corner of the block, then compare it to doing that with other woods. Another way is to see how heavy it is compared to similar sized woods. The grain pattern on the wood is a very common type, but it looks a little like walnut.
One other thing you can do to see if it would stabilize well is to cut part of it, and see how shiny (oily) it is. Oily woods do not stabilize well.