Black is one of the many colours of tourmaline. However, since it is pleochroic, it might be green when viewed along a different axis. This crystal is also piezoelectric which means that through mechanical action an electric pulse is formed, similar to quartz.
As a healing stone, black tourmaline is said to protect against cell phones and radiation. It grounds energy, clears negative thoughts and increases physical vitality. It is said to strengthen the immune system, treat arthritis and provide pain relief.