Crystal sphere

A crystal sphere, sometimes called crystal shell, was a gigantic spherical shell that contained an entire planetary system. Each sphere varied in size but typically they were twice the diameter of the orbit of the planet that was furthest from the sun or planet at the center of the sphere (the system's primary).

Description
Crystal spheres were made of a dark ceramic material that had no gravity and appeared to be impossible to damage by any normal or magical means. They always looked the same when seen from the outside.

The surface of the sphere was called the "sphere wall" and separated the void of wildspace (within the sphere) from the phlogiston (which surrounded and flowed outside the sphere). Openings in the sphere wall called "portals" allowed spelljamming ships or wildspace creatures to pass through and enter or exit from a crystal sphere. Portals could spontaneously open and close anywhere on the sphere wall. Magical spells (or magical items that reproduced their effects) could allow a portal to be located. Other magic could open a new portal or collapse an existing one. Ships or creatures passing through a portal when it closed could be cut in two.

Crystal shells also marked the outer limit of a deity's influence. Divine magic that emanated from a deity worshiped on a world within a crystal sphere was severely dampened or completely non-existing once one crossed the crystal shell into the phlogiston and into other crystal spheres, unless the deity also enjoyed considerable worship in the new location.

All known crystal shells in the phlogiston were spherical. The reason for this was unknown, but several hypotheses were mentioned by Elminster in a correspondence sent to Khelben Arunsun. Possibilities included the naturalness of a spherical shape in a fluid (akin to bubbles), practicality and ease of construction, and the balance of gravity produced by a sphere. Elminster also speculated on the possibility of different shapes in unexplored reaches of the phlogiston, such as cubical, ellipsoidal, triangular, irregular, and even toroidal shells.

History
The origin of the crystal spheres was unknown. It was conjectured by legends and some sages that they had been created by deities to protect worlds in their care from the phlogiston. Others believed that crystal spheres were even older than the gods and were designed to contain their activities and to keep them localized.

It was believed that Realmspace was among the oldest of the known crystal spheres. Many scholars believed that crystal spheres warmed up as they aged and, since Realmspace was slightly warmer than other crystal spheres, its age was posited to be greater as well.

Notable Crystal Spheres

 * Realmspace, the crystal sphere that contained the Torilian solar system.
 * Greyspace, the crystal sphere that contained the Oerth solar system.
 * Krynnspace, the crystal sphere that contained the Krynn solar system.
 * Clusterspace, a remote crystal sphere that contained the Astromundi Cluster, regarded by some sages as the place where mind flayers originated.