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.

Sources

Tumkeoite is not naturally-occuring in the Demiplane of Dread. It can only be found in rare ingots traded by the Vistani, certain dungeon drops, within the Mists, or by smelting existing items containing it.