Stormlight

Stormlight is a form of energy on Roshar. When a highstorm passes, gemstones left out in the tempest absorb its power and afterward glow steadily with a light that neither burns nor flickers. Rosharans mount these gemstones in glass beads called spheres, which serve the eastern kingdoms as both lamplight and money: a sphere's value depends on the kind and size of the gemstone at its heart, and a bowl of them lights a room more cleanly than any candle. The glow is not permanent. Infused spheres slowly leak their light and go dun, so people renew them by setting them out in baskets and cages during the next highstorm. At the start Stormlight is a fact of daily life, the currency and lamplight of a stormy world.