Sunstone

Between the Crimson Was and the Brightening, the Order of Luminance was at their strongest. However, there is an inherent problem with harnessing sunlight as magical power; nighttime meant less power, so the wizards (now called Brightcasters) began researching ways to store their energy somehow. Eventually, they developed sunstones. These crystals are mined deep in the mountains. They are pale yellow in color and are semi translucent. Great care is taken when extracting them, as they are fragile. Gem cutters in the mountain region specialize in shaping the crystals.  They function as both a focus for magical power and as storage. If left in the sun, these gems will absorb unfocused sunlight, keeping it for later use. 
They are also used to continually power enchantments, such as the protective field used during the Day of Luminance. Many Brightcasters incorporate a sunstone into their magical foci or jewelry. Staves with sunstones mounted to the top, amulets, and rings are most common, but some have attached them to muskets or more modern equipment in order to channel energy into the item.
The Sunlit Mountains are known for having the richest deposits of sunstones. Miners and gem cutters in the region strongly rely on them as a trade resource with the Brightcasters, who offer magical protection against the sunbeasts in exchange. If a deposit runs dry, entire towns sometimes uproot themselves to merge with other nearby settlements rather than risk their livelihood.
Often, a town will trade with only one or two local wizards. Before the Brightening, towns could sell to the highest bidder because there were far more Brightcasters around. Now, selling to a non-local runs the possibility of having your town wiped out by jealous wizards. The constant threat of sunbeasts and the necessity of magical protections ensures that a settlement would not survive without refining sunstones.