Black Obsidian is known to aid in detoxification of the mind, body, and spirit.   It can help to release fears and patterns.  It is often used for protection against negative energy.  It is said to shield against psychic attacks, negative thoughts, and energy. It is a strong psychic protection stone that dissolves emotional blockages and absorbs negative energies. 
 
Approximately 7/8" - 1 1/8"
From India.