Ungulcah Crystals

Within the caverns of Isolabirre, the Drow Elvirim make use of Crystal Lanterns to provide the wan light they need to see.
The crystals that provide the otherworldly purple light for the lanterns are known as Ungulcah Crystals. They are radioluminescent, so their light never fades, or fades so slowly that no one has noticed that they have faded, even within the long lives of the Drow.
The crystals are found in the caverns of Isolabirre, especially in the Deep Chambers and the Far Depths. From there, the Drow Elvirim of Isolabirre harvest a few every year to make new lanterns to replace any that are broken. If they are found elsewhere, no one knows. No Dwarvirim miner has ever claimed to have found any.
Though the Drow, of course, do not know it, nor does anyone else on Derkomai, the Ungulcah Crystals constantly emit low levels of radiation. Long-term exposure to the radiation from the crystals has caused mutations to Drow Elvirim genes, which has led to their extreme sensitivity to sunlight and darkened their skin, but has also made them resistant to radiation and magic damage and has given them the ability to see well in darkness so deep that most surface dwellers would say is nearly pitch-black.
Because Ungulcah Crystals are found only in the caverns of Isolabirre, they are extremely valuable outside of Yfel. Miners and adventurers covet them, as they provide a constant reliable light source without requiring any spell or light-producing Omphalos. However, finding one for sale anywhere is next to impossible, and when one is available, it commands a very high price.

A large Ungulcah Crystal by Peter Nelson (Zero Sum Games)

A large Ungulcah Crystal

Drow Crystal Lantern by Peter Nelson (Zero Sum Games)

A Drow Crystal Lantern