A crystal found in Starfleet subcutaneous transponders, which Kirk and Spock dig out and turn into a crude laser to escape from a cell on Ekos.
Rubindium crystals appear in the Original Series episode Patterns of Force. Before beaming down to Ekos, Kirk and Spock have transponders injected under the skin of their arms so the ship can find them.
Captured by the planet's Nazi-style regime, they are flogged and locked up. Spock proposes using the rubindium in the two transponders: placed the correct distance apart and fed with light from the cell's lamp, the crystals should act as a simple laser.
They cut the devices out with a strip of metal from the bunk, and Spock climbs on Kirk's lash-marked back to reach the bulb. The beam burns out the lock and the two escape to join the Zeon underground.