Suspendium is a teal-coloured crystal used to keep airships aloft. Suspendium doesn't actually cancel out gravity. Rather, Suspendium crystals repel one another when charged. In the world of Airships, Suspendium is absolutely everywhere: there’s big deposits in the rocks, it’s in the soil, in plants and animals and people. It’s even in the air as fine Suspendium dust. Because there's a lot of Suspendium in the ground, charging a large crystal causes it to experience an upwards force that diminishes with distance from the ground. Due to this diminishment, ships have a service ceiling. A single crystal, as long as it’s flawless, does not repel itself. This is because the repulsive force is a function of the alignment of the crystal structure: two crystals that are perfectly aligned also experience no repulsive force.
Big crystals are rare. Most Suspendium is in small, useless crystals, sand, or dust. Suspendium dust will floats above the ground in response to the smallest charge in the air. In some areas, you can tell approaching thunderstorms by the dust that comes up.
When ingested, Suspendium is not toxic, but it does tend to bio-accumulate, staying in the body. And since all those little bits of Suspendium repel each other, they exert mechanical stress on the cells they’re in, leading to illness and frequent cancers. Some people are unlucky and have an especial affinity for accumulating Suspendium. They tend to die pretty young, though a careful diet can ameliorate this.
Suspendium is a mixed blessing. It makes travel much easier: airships, cheap Suspendium barges floating just above the road. On the other hand, it contaminates much of the world, making vast areas unsuitable for agriculture, shortening lifespans and being a general nuisance. And it enables and encourages war for those parts of the world that are fertile and productive.