Villium

Villium is a crystal that commonly forms within the Underdark due to the cold and sunless conditions within. Sometimes nicknamed "Hands of Vörd" due to its appearance and lore, it often looks like small light blue quartz-like crystals that form in seemingly random tower structures.
In its raw form, it is relatively dense and hard to mold. However, under extreme cold, it begins to take on a more gum-like elasticity. The duergar that roam the Underdark are well-known to work with villium often, and create entire structures and citadels out of the powerful material. Using it as a focus for abjuration magic allows the spells to be more resilient and more powerful, acting as a shock-absorption material. When molded into armor and buildings, it provides an extremely tough outer shell that not even powerful drills can break through.
Obtaining villium is difficult for most people to obtain. Believing it to be formed from the blood of their elder god, Vörd, convincing them to part with the crystal is much easier said than done. Well experienced thieves might sneak into their strongholds and steal the precious crystals, whereas others might receive the highest honor of being given some of the villium in exchange for heroic deeds in the eyes of the duergar. Those who do obtain it either sell it for a high price to scholars, or use it to craft new spells of their own.