We use black walnut as a lining for these zebrawood rings. A little more about that wood:
Black walnut (Juglans nigra) is a flowering tree that grows in much of the US and produces (unsurprisingly) walnuts. This lumber is prized for its dark and strong heartwood.
Walnut trees produce an herbicide called Juglone that can kill or stunt the growth of some nearby plants and is toxic to some trees such as birch.