The most essential and valuable commodity in the universe, melange is a geriatric drug that gives the user a longer lifespan, greater vitality, and heightened awareness; it can also unlock prescience in some subjects, depending upon the dosage and the consumer's physiology. This prescience-enhancing property makes interstellar travel possible. Melange comes with a steep price however: it is addictive, and withdrawal is a fatal process.
In Dune, Lady Jessica notes that her first taste of spice "tasted like cinnamon." Dr. Yueh adds that the flavor is "never twice the same .. It's like life — it presents a different face each time you take it. Some hold that the spice produces a learned-flavor reaction. The body, learning a thing is good for it, interprets the flavor as pleasurable — slightly euphoric. And, like life, never to be truly synthesized."
Herbert notes in Dune that a pre-spice mass is "the stage of fungusoid wild growth achieved when water is flooded into the excretions of Little Makers," the "half-plant-half-animal deep-sand vector of the Arrakis sandworm. Gases are produced which result in "a characteristic 'blow,' exchanging the material from deep underground for the matter on the surface above it." Liet-Kynes describes such a spice blow in Dune:
Then he heard the sand rumbling. Every Fremen knew the sound, could distinguish it immediately from the noises of worms or other desert life. Somewhere beneath him, the pre-spice mass had accumulated enough water and organic matter from the little makers, had reached the critical stage of wild growth. A gigantic bubble of carbon dioxide was forming deep in the sand, heaving upward in an enormous "blow" with a dust whirlpool at its center. It would exchange what had been formed deep in the sand for whatever lay on the surface.
Herbert writes that the pre-spice mass, "after exposure to sun and air, becomes melange." He later indicates its color in Children of Dune, when Leto II passes "the leprous blotches of violet sand where a spiceblow had erupted."
Collecting the melange is hazardous in the extreme, since rhythmic activity on the desert surface of Arrakis attracts the worms, which are four hundred meters (1,300 feet) in length on average, and very dangerous, capable of swallowing a mining crawler whole. Thus, the mining operation essentially consists of vacuuming it off the surface with a harvesting machine until a worm comes, at which time a carry-all aircraft lifts the mining vehicle to safety. The Fremen, who have learned to co-exist with the sandworms in the desert, harvest the spice manually for their own use and for smuggling off-planet.
Later in the series, an artificial method of producing the spice is discovered by the Bene Tleilax, who develop in secret the technology to produce melange in axlotl tanks. Still, the technology is not fully successful in pushing natural melange out of the marketplace.
Although it is referred to as "spice" and can be mixed with food, melange is indeed a drug in the clinical sense, its use being physically addictive and having intense psychotropic effects. Spice is also a powerful entheogen, which suitably trained adepts can use to initiate clairvoyant and precognitive trances, and access racial memory. A melange user, once addicted, is thereafter compelled to continue using it for the rest of his or her life, as any discontinuation of its use will induce excruciating withdrawal symptoms, and if not quickly resumed, will invariably be followed by death. Taken daily, however, melange can extend its user's lifespan by hundreds of years. Due to its rarity and value, and its necessity as a catalyst for interstellar travel, the group controlling spice production on Dune controls the fate of the Empire, a form of hydraulic despotism.
The taller of the two, though, held a hand to his left eye. As the Emperor watched, someone jostled the Guildsman's arm, the hand moved, and the eye was revealed. The man had lost one of his masking contact lenses, and the eye stared out a total blue so dark as to be almost black.
When aerosolized and used as an inhalant in extremely high dosages — the standard practice for Guild Navigators — the drug acts as a mutagen. In the first chapter of Dune Messiah, Guild Navigator Edric is described in his tank of spice gas as "an elongated figure, vaguely humanoid with finned feet and hugely fanned membranous hands — a fish in a strange sea."