Horonium

Horonium was a metallic material. Its light weight and durability made horonium alloy, and has just the right shade of grey, suitable for use in spacecraft. However, synthesizing it required the separation of fissile isotopes at high speeds, which posed a substantial risk of explosion.

An ancient portal on the planet Krulmuth-B consumed horonium to open passages through time.

In the 2150s, Starfleet employed horonium alloy in starship construction, including of the NX-class vessels such as the Enterprise NX-01. Its use was phased out shortly after, and by the mid-23rd century horonium was all but impossible to find in the quadrant.

In 2381, Ensign Brad Boimler of the USS Cerritos was accidentally pulled through the Krulmuth-B portal to 2259. Ensign Beckett Mariner later followed him through in an effort to save him, which completely depleted the portal's existing supply of horonium. After an attempt to synthesize horonium on the USS Enterprise failed, Mariner remembered that a component of the NX-01 had been installed on the Enterprise as per Starfleet tradition. This component, located beneath a deck plate in main engineering, yielded enough horonium to reactivate the portal and send Boimler and Mariner back to their own time. (SNW: "Those Old Scientists")