Black obsidian is reflective and absorbing, making it a good choice for protection. It can however be harder to work with than some of the other black stones, because of its mirroring attributes. This means that it is very effective at reflecting back energies, both to the sender - if undesirable - and to the self, although this can be useful if introspection and examination of ones own motives is required! It is also a very good choice as a grounding stone, as it will help keep the Earth-connections clear during the process.