Eberron

Eberron was a planet somewhere.

Cosmology
Eberron was a part of the Great Wheel cosmology and the overall multiverse, being situated in the Deep Ethereal. However, there it lay at the center of its own material plane and its very own cosmology of other planes that orbited it and had profound effects upon it. Furthermore, this cosmology and its Ring of Siberys sealed the world away from the regular Great Wheel planes, preventing outside deities and powers both celestial and fiendish from having an influence there.

Nevertheless, it was possible in theory to travel to and from Eberron via the Ethereal plane and other spells intended for traversing the planes, though the cosmology was designed to prevent it, or at least make it very difficult. Alternate material planes such as Eberron's might also be reached by going deep into the Plane of Shadow or through a shadow portal to the Plane of Shadow.

It's unknown if Eberron had a crystal sphere within the phlogiston.

History
Eberron and its unique cosmology were created by three great progenitor wyrms. They designed a new cosmic system, made a new planet, and populated it with recreations of the dragons, elves, orcs, and other races of the multiverse. However, they intended that they evolve outside the influence of their respective deities like Corellon, Lolth, and Gruumsh and others.