Obsidian is formed through volcanic action, heating and compressing silicates. Rainbow Obsidian is a natural black obsidian that, when polished and exposed to a strong light, shows bands of various rainbow colors. It is said to be the stone used by Nostradamus to foresee future events, and used by John Dee for skrying, and has been used to sharpen the senses and for connecting to psychic realms. Rainbow Obsidian is an excellent grounding stone and is also a protective stone that keeps one safe from emotional and physical harm. It provides a shield from negativity.