Burr Oak is a natural process where small twigs near the base of the tree are removed, usually by grazing animals. As this is repeated, the tree forms a mound as it thickens in the area and this is the burr. You will see the areas where the twigs attempted to grow and have formed amazing swirls and patterns. The fissures around them can sometimes result in open grain, which I have filled with an inlay of bronze.
With the symbol burned into one surface, this means it will not wear off after many years of use.