Sand

Sand was a fine substance composed of rock and mineral. Sand could be made into glass, and would eventually do so if left alone for long enough. (TV: "The Sea of Death", The Caves of Androzani)

Having passed through a wormhole to the planet, the Tenth Doctor explained that the "sand" on San Helios was in fact the waste left by the Swarm after they laid waste to everything on the surface. The Doctor tasted this "sand" and was able to judge its composition by doing so. (TV: Planet of the Dead)