The Phlogiston

"The Phlogiston, The Rainbow Ocean, The Flow - an endless sea of every color" - gnome astronomer Hub-El
The phlogiston is that which is outside and between crystal shells. It appears as an infinite fog of swirling colors.
The Flow's vastness is dominated by Currents, which link crystal shells in a mind-bogglingly gigantic 3-D web. Flowrivers are useful to Star Sailors because they (greatly) facilitate travel between the crystal shells. In many cases, a journey directly to a specific crystal shell could take many years, but the yoyage can easily be shortened to a much smaller amount of time by following the Currents from one shell to the next. (Hence, navigation in the phlogiston is a tricky connect-the-dots activity).
It is not possible to see a Current (though detailed measurements can detect a significant increase in the drift of the multi-colored fog in one or two directions). They are felt more than seen; an individual sitting on a spelljammer helm can easily realize that a starship has entered a Current.
Most Currents are two-way, allowing passage in either direction between two crystal shells. However, some are one-way, so it is not advisable to try to double back along a one-way Current (doing so poses the same difficulties as not using a current at all)
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