Occasionally one can find natural crystals of pure Vis deep underground. These slowly grow over time into larger crystals.
They grow much faster in areas where the aura has a positive charge. The crystals themselves have no effect on the local aura, unless the aura in the area becomes extremely low. In that case trace amounts of vis are released from the crystals which may cause the aura to replenish at a slow rate.
Vis crystals have a myriad of uses, including seals, infusion and crafting recipes, and research 'subjects' to name just a few, which makes them a sought after resource. Care must be taken not to over-harvest them however, since doing so causes instability in the local aura which might lead to an increase in taint level. Without the crystals the aura also cannot replenish should it drop to low levels. A Crystalline Bell is a good way around this problem.
There are six different types of crystals:
It is also possible to craft a placable version of the vis crystals if you have harvested enough of the individual crystals. As long as it is placed on a solid surface it will grow and behave as a normal crystal.