Obsidian is a natural, volcanic glass.

Obsidian is a very protective stone,  and is excellent for removing negativity. It is also excellent protection against psychic attacks. 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.   Note that healing crystal meanings are spiritual supports to healing and are not prescriptions or healthcare information.  Obsidian is related primarily to the root chakra, though differing colors may relate to different chakras as well.
2-3cm stones