Sunstone Crystal

The inside of the Fortress of Solitude, made from Sunstone Crystals. One of the larger crystals is projecting a holographic playback of Jor-El.

Sunstone Crystal is a type of crystal used on the planet Krypton, both as a building material and as a form of computer system. Several incarnations of the Fortress of Solitude are made mainly from the crystals, or otherwise incorporate them into its design. Many structures in the city of Kandor, and now on New Krypton, are made with this material. (Superman: The Movie, Superman Returns, Comics)

The crystals are activated upon exposure to water, whereby they begin growing, sometimes forming structures. They may also take on the properties of other stones near them at the time they begin growing. (Superman Returns)