The Ancient Onyx is a large, black colored Chaos Emerald located somewhere in the southern hemisphere near the Mystic Ruins in the Mysterious Cat Country. It and a group of Chaos Emeralds nearby were worshiped by a group of Chao and protected by the water creature Chaos. After Chaos went rampant and wiped out the Knuckles Clan, the Seven Server Chao absorbed Chaos' energies, transforming into the seven Super Emeralds, while Tikal then imprisoned both herself and the weakened Chaos in the Ancient Onyx. Hundreds of years later, Dr. Eggman cracked open the Black Emerald, thus releasing Chaos from his captivity along with Tikal. From there, Dr. Eggman unleashed Chaos as his personal water demon, nearly destroying Station Square. After Super Sonic and Knuckles worked together and defeated Perfect Chaos, the water demon and Tikal returned to their imprisonment in the Black Emerald. Additionally, the Super Emeralds transformed back into the Seven Server Chao, who went with Tikal and Chaos into the Black Emerald.
The Ancient Onyx later became a point of contention when tensions rose between the Felidae and the Wolf Pack Nation, both of whom claim the gem to be a sacred object in their respective cultures and rightfully theirs. This eventually led to an armed confrontation between the two parties when the Ancient Onyx went missing, with Sonic the Hedgehog and Sally Acorn caught in the crossfire. Fortunately, the culprits were soon exposed as a chapter of the Dark Egg Legion composed of former members of both communities, hiding out in the Mystic Ruins. After driving out the enemy, the two groups were convinced to share custody of the Ancient Onyx by Sally, who along with Sonic and Big the Cat were inducted into the Wolf Pack with their hands on the Emerald. Sally also voiced her concerns that the artifact resembled a Chaos Emerald, but Sonic was quick to dissuade her of the notion.