A crystal sphere 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).
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). The sphere wall had no gravity and appeared to be impossible to damage by any normal or magical means. 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.