Obsidian is a very unique rock due to its smooth, uniform volcanic glass texture. Obsidian is usually considered an extrusive rock, because it usually solidifies above Earth's surface where the edges of a lava flow come into contact with cool air or water.
 
This stone is strongly protective; it absorbs negative energy and stimulates growth on all levels. In particular obsidian protects the gentle from abuse, and is used to cut attachment cords to release one. It is a very grounding stone, and  very healing. Obsidian is also an excellent manifestation stone. 
 
Physically it is said to benefit the stomach, intestines, muscle tissue, and can rid one of bacterial or viral infections.  It sharpens and focuses internal and external vision, and helps get in touch with buried issues before they explode.   
 
Obsidian is related primarily to the root chakra, though differing colors may relate to different chakras as well.