Black Agate Bracelets: The Dark and Enigmatic Stone Black Agate, a unique chalcedony stone formed in volcanic rock, is part of the Agate family. The countries that use these dark and mysterious stones the most are Mexico, Brazil, the United States, Uruguay, and Madagascar. A black agate ring with a glossy finish is enigmatic and mysterious, with the darkness only lessening when illuminated, revealing its true transparency. The central meaning of black agate is strength, emotional harmony, and stability.