Altar of the Emeralds

The Altar of the Emeralds is an area within Sonic's dimension, located not far from Leaf Forest. It is a mythical ruin which houses the Master Emerald, along with the seven chaos emeralds. Often, Knuckles the Echidna can be found here, keeping watch over the Master Emerald in particular. In the "Dimension" saga, however, his preoccupation with Sonic, Tails, and the newly-formed alliance of individuals from alternate dimensions forces him to abandon his usual post, which allows Doctor Eggman to later steal the Master Emerald and keep it aboard his Death Egg—though this is hardly the first time the Doctor has attacked the Altar. Five years after the War Above Earth, Gemerl (under the influence of GAMP the supercomputer) visits the Altar alone, where all of the emeralds have since been returned to their rightful place; however, only the chaos emeralds are taken—as Calvin and Hobbes later discover for themselves, upon seeing the Master Emerald as the sole remaining gem sitting atop the Altar.

Trivia

 * In the official Sonic game(s), the landmass on which the Altar of the Emeralds resides is known as "Angel Island," which is usually seen floating in midair via Master Emerald. The "Dimension" saga does not refer to this island by name, nor is any reference made to the island's airborne nature.