Tumkeoite

Tumkeoite is a foreign metal from Oearth. It is a rare, heavy, and unstable magnetic mineral metal that slowly decays with exposure to air. It radiates light in both the visual and ultraviolet spectrums as it slowly decays (ultimately transforming into a mineral called lacofcite). To human eyes this light is a ghostly plum color, but to those who can see into the ultraviolet (as can drow and derro), it appears a shimmering amethyst hue. The resultant radiation caused by this decaying interferes with teleportation.