Darkstone

Darkstone is a magical mineral, often paired with sunstone because of how their properties work together. It is exceedingly durable, and it defies fundamental laws of the universe, like gravity and conservation of momentum. If light isn't shining on darkstone, it becomes fixed in place, presumably relative to Everfall's main darkstone core. If it is moving or being transported and the light exposure stops, the velocity of the darkstone will stop as well, thus remaining affixed in the point in space where it last received light, even if it is in midair. It takes incredible amounts of force to move a darkstone if it lacks light. A darkstone is pushed away from a directional source of light, but moves freely if completely surrounded by light or in contact with a lightstone

Due to the way it is pushed away from lights people have constructed darkstone vehicles and skyships using darkstones for flying and accelerating.

Sunucles shatter when they come contact with darkstone the same way as they do with the Shade. Some lenses in sunforges are made of darkstone. Darkstones also nullify the ability of lightbinders and, presumably, lightbringers as well.

Just like with sunstone, passing through the barrier of the universe while holding a darkstone causes death.

trivia

 * On pages 236-239 in Shadow of the Conqueror the way darkstones function in a skyship's core is explained, including luminance threshold.