Scrith, forms the walls and floor of the Ringworld.
Scrith is milky-gray translucent in color, and is a nearly frictionless material. The relatively thin layer of scrith that forms the floor of the Ringworld blocks the passage of 40% of the Neutrinos that encounter it, equivalent to almost an Earth-lightyear of lead. It also absorbs nearly 100% of all other radiation and subatomic particles and rapidly dissipates heat. The tensile strength of scrith is theorized to be similar to the Strong nuclear force, with the Ringworld foundation only measuring about 30m (100 ft) thick.
It was established, in The Ringworld Engineers that Scrith is not transparent to magnetism, otherwise Louis couldn't have used magnetic fields to add extra fuel to the ramjets to help save the Ringworld.
Due to its enormous strength, scrith is impervious to most weapons. A body (such as a comet or asteroid) striking with enough kinetic energy may be able to deform the Ringworld floor enough to puncture it. Read: Fist of God The Ringworld engineers used a device, called the Cziltang Brone in the City Builder language, to pass from the vacuum of the rim spaceports through the scrith to the habitable surfaces of the Ringworld.
The physical composition of scrith is unclear, but it appears to share some of the properties of a metal; for instance, the high tensile strength, the ability to quickly conduct heat, and the ability to retain an induced magnetic field. Scrith is said to have been artificially produced through the transmutation of all or nearly all of the star system's matter.
Combining aspects of the Pak building material Twing, itself an incredibly durable programmable substance, with the theory of resonant molecule bonds could produce something akin to scrith. Most notably as both scrith and twing can be reshaped to pass through thanks to a softening device, most likely similar to the Cziltang Brone of the City Builders.