Rubindium

Rubindium is an element which is generally found in crystalline form. While finding stable rubindium was a rare discover, in the 23rd century, rubindium crystals were used by Starfleet as part of their subcutaneous transponders.

While imprisoned on the planet Ekos in 2268, Commander Spock was able to create a powerful laser using the crystals in his transponder, and was able to escape from prison. (TOS episode: "Patterns of Force").

In the 2290s, the Federation discovered large quantities of stable rubindium on one of the inner planets of the star system surrounding Orelt. After lengthy negotiations with the Frunalians, the Federation were able to gain mining rights. (TOS novel: Crucible: The Fire and the Rose).