Sunstone

Magical mineral that acts as a light sponge. Soaks in light while it's bright, and releases it when the ambient light levels are below its saturation.
Commonly used in the northern forests and taigas to create wards against phantoms.
Naturally occurs in large chunks partially exposed to air.

Artificial Sunstone

Synthesized sunstone has a more frosty look to it, as opposed to the clear crystal of natural, due to the fact that it's essentially re-compressed fragments of dust. Higher quality artificial sunstone comes closer to the clarity of natural, but still has worse efficiency.
The discovery of synthesized sunstone around 100 AoW (some 400 years after the Citadels' original founding) unlocked the floodgates for the northern territories to truly expand, where before they were bottlenecked by the amount of territory they could safely cover with mined sunstone.