Feathers & Glass
Viz was formed by the explosion that tore the continent apart, the pressure forcing some of the magical energy into a solid state, almost like diamonds. Viz is useful because it can be used to power spells – simply scratching it’s surface is enough to disrupt it’s solid structure and revert it to energy. It’s volatile and unstable.
To the layman, Viz is magic made solid. Scratch its surface and for a one-time boost of MP to power a spell or anything else you’d need MP for.
Salt has quite an abundant supply of loose Viz, though no one has tried to export it yet.