Deep below the ground within the Mystic Cave Zone is an area that looks similar to Angel Island's very own Hidden Palace Zone and is lined with thousands of green-colored gems, many of which bare an uncanny resemblance to the Master Emerald. The cave itself appears purple and dull.
After defeating Dr. Eggman and his assistant, Dr. Julian Snively, on South Island, Sonic Hedgehog chased the two mad doctors down to Westside Island, where he teamed up with his new sidekick and best buddy Miles "Tails" Prower as well as the Freedom Fighters. Mystic Ruins Zone was one of the Zones they passed through in their efforts to defeat the mad doctors, whose goal on Westside Island was major construction and refinements as part of their plan for world domination.
Knuckles, Amy, and Team Dark first came across the cave while tracking the source of a unique energy signature. Descending into the caves depths, the group entered a part of it made mostly of gemstone and crystal, much to the delight of Rouge the Bat.