Black onyx is a protective stone. It is said to chase away nightmares, dispel negative energy, increase confidence and self-esteem, and enhance concentration. It is thought to balance emotions and to help us to keep feelings under control. Black onyx is believed to heal old wounds and to help us to change old habits. Black onyx is associated with the Zodiac signs of Leo and Capricorn. It is the mystical birthstone for the month of December, and an alternate birthstone for the month of February. It is given as an anniversary gift for the 10th year of marriage.