Obsidian is an igneous rock, often called volcanic glass because it is formed from molten lava that has cooled very quickly. Black is the most common colour but it can also be brown, or a natural blend of both which is referred to as Mahogany Obsidian.
Black & White Snowflake Obsidian
Brown & Grey Snowflake Obsidian
In general, 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.