The Vis Crystal is a group of items added by Thaumcraft 6. They are used to craft various items in the mod. The primal Vis Crystals are used in the Arcane Workbench to craft arcane items. They are possible items to be submitted in the Research minigame.
On occasion there is so much primal energy in the aura that it will settle into rock from which vis infused crystals will grow. As long as they are fed vis they will continue to grow and even spread. Once the aura dips low enough the energy once again dissipates out of the crystals, replenishing the aura and causing the crystals to crumble. In effect, they are natural, or in this case, supernatural vis batteries. They will never crumble away completely and at least a small remnant of the crystal will remain. These crystals can be harvested for elemental shards that have many uses in Thaumaturgy.