Crystal Sphere

A crstyal sphere is a massive barrier that seperates Wildspace from the Phlogiston of the Astral Sea. Within each crystal sphere is a solar system, consisting of at least one star and one or more orbiting planets. The size of a crystal sphere is directly related to the size of the internal solar system. Each crystal sphere appears to be made of an unbreakable, dark ceramic material, obscuring the system inside.
Although crystal spheres are solid and large, they contain no gravity and no atmosphere and no life has ever been noted to live on the outside of a sphere.
Within each crystal sphere is a both a unique solar system physically but also metaphysically. Many crystal spheres are under the influence of different gods, different rules of magic, and are influenced by different planes of existence. Spheres that are relatively similar appear to cluster close to one another, and the further away a creature travels from their home sphere the more differences become apparent.

Piercing a Crystal Sphere's Shell

Ever since crystal spheres were discovered, sentient creatures have desired to travel through them. While there are numerous spells that allow one to travel to an different plane of existence, abilities that allow one to travel through a crystal sphere and appear on the exact opposite side are rarer.
A few of the known methods to travel from one side of a shell to the other are as follows:

Teleportation Spells

While transdimensional teleportation is blocked by the Phlogiston, intradimensional teleportation allows one to travel in to and out of a crystal spheres. Examples include Teleport and Dimension Door.

Spelljamming Helms

Vessels equipped with a spelljamming helm can turn part of a crystal sphere immaterial, allowing for the vessel and its occupants to travel through the crystal sphere's shell. The immateriality lasts only as long as it takes for a spelljamming vessel to move through.

Naturally Occuring Portals

Most crystal spheres have natural portals that form at random times and at random locations. For most creatures, these portals are only discovered by happenstance and are considered too unreliable for regular travel.

Stars

Some crystal spheres have a star or stars that overlap with the border of the sphere and can be traveled through to bypass the sphere's shell. However, the extreme heat of stars, the fact that many stars are portals to the various Elemental Planes of Fire, and the near constant explosions of the Phlogiston that comes in to contact with the extruding star makes such travel dangerous to all but the strongest or more fire resistant creatures.

Innate Abilities

Some large, space-faring creatures, such as the various space dragons, have the innate ability to create portals that allow them to traverse in and out of crystal spheres. This ability is also shared by the Spelljammer. Portals created in this way close much slower than other, non-naturally forming portals, allowing creatures or ships trailing behind to make use of them.

Origin Theories

The origin and true purpose of the crystal spheres is a hotly debated topic from the halls of academia to the taverns of jammers. Though various theories wax and wane in popularity, the six below theories have proven to be the most popular.

The gods created everything

When the planes of existence were created, the gods divided chaos and order as the Astral Sea and the crystal spheres respecitvely. The Flow was then created in turn to connect the crystal spheres with one another, forming a unified Material Plane. The differences between the crystal spheres are a result of various gods having different ideas of how to manage their realms.

Over-gods created everything

Even the gods have creators, and the gods-of-gods created the planes of existence, the Astral Sea, and the various crystal spheres. Once creation was established, the over-gods created the known gods and placed them throughout creation.

The Flow predates everything

Before even the gods existed, there was the Astral Sea and the Phlogiston. The crystal spheres were created and were placed throughout the Astral Sea to harness the Flow. Over time, the crystal spheres will grow in size and merge together, eventually consuming the entirity of the Flow. The result of or the reason for this endgame is unknown.

The gods fear the Flow

The crystal spheres were created because the gods fear the Flow and existance in the Astral Plane carried too much risk of coming into contact with it.

The gods fear mortals

The crystal spheres were created as cages to keep sentient creatures contained.

Both the Flow and the crystal spheres predate the gods

The planes of existence, the Astral Sea, the crystal spheres, and the Flow all existed as they are now, and then the gods moved in and took them over.

Type

Metaphysical, Arcane