One size fits all with elastic band filled with drilled tourmalines. I have to say, my favorite one for psychic protection and grounding! I need to keep one always near me...Black Tourmaline keeps an electric charge due to its pyroeltric features (meaning it can generate electricity when heated), as well as piezoelectric (meaning it can store an electrical charge), causing it to release negative ions and far infrared radiation - these are very beneficial for health. This incredible stone is associated with protection, grounding, health, happiness, luck, and positivity. Black tourmaline has been used since ancient times by Shamans and healers for protection. Black Tourmaline is considered by many to be the most powerful protective stone and it is one of the only stones that protects and heals on all levels - physical, emotional, mental and spiritual.