The formation of pure onyx is a complex process that involves the interaction of geological and chemical factors. The process begins with the deposition of silica-rich solutions into cavities and fractures in rocks, where they precipitate out of solution as chalcedony. Over time, the chalcedony layers build up, forming a banded or layered pattern. The unique conditions required for the formation of pure onyx, including a stable silica-rich environment and a lack of impurities, make it a rare and valuable stone.