Called the 'mirror stone', it reflects ones personal shortcomings such that they can come to the conscious to be dealt with, then let go of.
The most well known of the different colours is Apache Tears and black obsidian.
Obsidian can help protect against the negativity of others and has a grounding influence helping one to stay focused in the moment. Grounds spiritual energy into physical plane. Brings an understanding of silence and 'the void'. Detachment, but with wisdom and love