Black Agate Necklace: The Enigmatic Stone  Black Agate, a unique chalcedony stone formed in volcanic rock, belongs to the Agate family. The countries that use these dark and mysterious stones the most are Mexico, Brazil, the United States, Uruguay, and Madagascar. When illuminated, a shiny black agate ring loses some of its utter darkness and reveals its transparency. Black agate's key themes are emotional harmony, stability, and strength.