Obsidian is actually a cold stone that comes from the most extreme heat of the heart of volcanoes from the magma. It is able to balance opposites.  
The North American Indians used obsidian a great deal to make cult objects, arrow tips and javelins and daggers for human sacrifices.  
It is generally used for psychic protection, cleansing of negativity, grounding and spirit communication.  It is a powerful eliminator of negative energies in oneself and one’s environment.  Like a psychic vacuum cleaner it cleanses the auric field of disharmony, negative attachments, anger, fear, greed and resentment. Element:  Earth.     
Chakra:    Root.