Protection, change, self-awareness.
The black agate represents protection, insight into unconscious aspects of ourselves and freedom from unfounded fear. Black agate can absolutely be worn as a talisman for protection against evil and access to one's own power grounded in self-awareness and relaxation. Like all agates, black agate is usually patterned. This pattern can give you hints about what you need to face within yourself to heal and stand up for yourself better. The black agate is a stone of change.