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Polyhedral Columbia
I was thinking about your suggestion to make Spelljammer phlogiston its own plane (in addition to ethereal, etcetera).
It can work. Essentially, the Phlogistonic Plane has to be moreorless the same thing as ‘hyperspace’. It has to be, because the ships must be traveling faster-than-light to reach the farflung worlds. In this phlogistonic hyperspace small distances within this plane equate to vast distances in the material plane.
Looking into ‘hyperspace’, I was disappointed that there is almost no reallife science behind it. It is mostly a technobabble plot device to get characters from one solar system to an other that is lightyears away. I was surprised there is virtually no science, because ‘hard’ science fiction writers invented hyperspace and utilize it in their stories. I was also surprised because there are comparable reallife science, such as wormholes and so on, to travel lightyears away.
Anyway, as a plot device a common trope is, strong *gravity* disrupts the hyperspace. So if a vessel gets too close to the gravity of a sun, the gravity yanks them out of hyperspace and into our space-time.
So for the purposes of D&D ...
From the perspective of being inside the plane of phlogiston, these solar gravity spheres look like ‘shells’ and direct contact with them immediately exits the phlogistonic plane and enters the material plane.
As such, these ‘crystal spheres’ are made out of the force of gravity.
Solar systems that are ‘sealed off’, like Dark Sun and Eberron, might do it by reversing the gravity that exists within the phlogistonic plane. So that while gravity works normally for a solar system within the material plane, the gravity pushes instead of pulls within the phlogistonic plane. This makes it difficult or impossible for phlogistonic ships to approach these ‘sealed’ spheres of gravity.