In 'Dune', the spice used for space navigation is melange, commonly called “the spice.” It is the most valuable substance in the universe because it allows the Spacing Guild’s Navigators to foresee safe routes through folded space.
Melange is produced only on Arrakis, the desert planet also known as Dune. Its existence is tied to the planet’s giant sandworms and their life cycle. Harvesting it is extremely dangerous: rhythmic movement attracts sandworms, which can destroy mining equipment and consume entire crews.
The spice has several other effects. It extends life, heightens awareness, and can unlock prescience in people with the right abilities. Heavy exposure turns a user’s eyes dark blue, known as the “blue-within-blue” or the Eyes of Ibad. Guild Navigators consume and inhale enormous quantities, eventually undergoing severe physical mutations.
“Spice” and “melange” are not competing answers: spice is the common name, while melange is the formal name. Water is especially dangerous to sandworms, and a sandworm is the source of the spice’s production—not the navigation substance itself. Frank Herbert introduced melange in the 1965 novel 'Dune'.