Vis is one of only two substances known to store magic; it is in itself pure crystalised magic. The substance forms in ore-like veins across the world and takes the shape of blue crystals, varying in shade from to light cerulean to a deep indigo, glowing in various intensities when filled with magic. It is prized for its ability to contain magic and durability. After being drained of a charge, Vis may be re-filled by any caster.
The crystals can be safely handled but are manipulable only by those with magic and are essentially immune to physical damage, which has made them popular for many applications from the industrial to the frivulous - cut into shape as long lasting jewellery, dissolved as super-permanent blue dyes and paint pigments, mosaics and carvings or powdered and mixed with alcohol to make the popular Vins Vinius drink.
However, its most valuable application is as a storage device for magic. It naturally contains magic when it is new, and therefore is a powerful source of energy, and one of only two materials available to magic users suitable for use as a battery. It is more common but less efficient than the other option, Pracina Wood.
Vis is primarily mined along the equator, where it is most common. No exact reason for why it forms in these locations in particular has been determined by mortals, though studies are being attempted to measure the magical content of the atmosphere at the equator versus other locations and whether this would contribute.