I also got one for ‘plotonium,’ but that may be taking the joke too far.
Koreite
Koreite (three syllables, by the way) is an ‘element’ that exists in universes that are at the outer edge of being habitable for ‘regular’ humans (i.e., us). If sampled, it appears to be atomically equivalent to indium… which is absurd, because indium doesn’t glow blue and is toxic. Koreite is also incredibly more common in its ‘host’ universes than indium is in ours.
Aside from the glowing and non-toxicity, Koreite is best known for its peculiar anti-radiation properties. The stuff absorbs radiation. Or alters it. Or outright transforms it… the bottom line is that in universes where koreite is not present ionizing radiation is varyingly dangerous, and will merely harm or kill you in fairly boring, unimaginative ways. In universes where koreite is present ionizing radiation ranges from being a mild annoyance to the reason why you’ve suddenly developed telepathy, radioactive breath, or a second head. Small doses of koreite will reliably act as a chelating agent, but if you drink more than a cup of it at one time you’ll never know what the end result is going to be.
There are a lot of organizations with access to other dimensions that are trying to create a way to import koreite for use in universes with normal, boring radiation. They have so far failed; which means that any medical clinics wishing to use koreite for its healing properties must set up physical shop in universes where exposure to plutonium can reliably swell an ant to the size of a Buick. And, really: at that point, the opportunities for adventuring scenarios write themselves.
Heh. Heh, indeed. Not only a good joke, but a fun trope to use in a SF RPG. Well played, sir, well played.