Obsidian is a very grounding and protective stone. It is used as a great cleanser of negative energies, helping to remove not only negativity experienced in the environment, but also that of one's own emotions, such as anger, jealousy, greed, resentment, etc.
 
Obsidian is often used for healing and releasing energy blockages, and has a tendency to work quickly to move truths to the surface to be resolved.