'Planarite' is terminology we realized we could use to distinguish between 'realistic' technology and magical technology, and to justify why the setting isn't overrun by airships or robots. The idea is for them to be a sort of resource that the Narrator can use as a Macguffin if someone wants to build neat steampunky tech, but which you can easily say, "Sorry, there's enough of it for this one cool machine, but not enough for society to mass produce this technology."
Like, normal matter is made up of protons, electrons, and neutrons. Planarite adds in other components (which I don't plan to fully codify because I'm not clever enough). Maybe it's other literal elements, like Time is literally an atomic element. Or more interestingly, stuff like tachyons (time), planckions (space), pyrons (fire), etc. are fundamental particles (akin to quarks and leptons and other stuff more complicated than what they taught me in high school). Certain types of matter might have a mix of normal baryons (protons, neutrons, electrons) and planons (the 8 elemental energies).
So by 522 AOV, the scholars of Lanjyr are realizing that by studying these base components, and learning to detect them, they can identify materials that can create magical properties. As chapter 3 points out, witchoil and firegems are planarite, and people have used them for a long time without really understanding the deep root level chemistry of them (or maybe the alchemy). There are other types too that are less common to find, or which aren't as easy to make.
As for your game, the Pemberton idea could work, though I think Tinker Oddcog fits better for the paradigm-shift level of innovation we're talking. Maybe in the ruins of the Ob facility in Cauldron Hill, Kieran can stumble upon a half-completed thesis that postulates an 8-planon framework, and which has identified the atomic structure of firegems and witchoil, with a hypothesis that others could exist too, and that they're actually the building blocks of magic.
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To be clear, I'd never use this sort of codified spell physics system in a lot of games, but I think it could fit the vibe of ZEITGEIST. Of course, it's also possible that this codification only really works while Jiese is around, and in other parts of the multiverse reality is a messy soup instead of a beautifully-faceted crystal.