Float-stone
From Tardis Wiki, the free Doctor Who reference
Float-stones were gravity repellent materials, formed from the axite ore found in the rocks on the planet Karagula. When fused together, the float-stones allowed the platform structures of the Karagulans to float about half a metre above the planet's surface. Smaller pieces, properly refined, made accurate ammo for hand-held catapults.
Lorton was a particularly good shot with a float-stone catapult. (PROSE: The Colour of Darkness)
