A rare mineral that is the sole antidote to deadly Rigelian fever, sought by the Enterprise on Flint's planet in the Original Series episode Requiem for Methuselah.
Ryetalyn is a mineral substance that, once processed, cures Rigelian fever, a disease that kills within a day or so. It is found on few worlds.
In Requiem for Methuselah an outbreak aboard the Enterprise sends Kirk, Spock and McCoy to the small planet Holberg 917G, where sensors show a deposit. The planet's reclusive owner, Flint, reluctantly allows them to stay and sends his robot M-4 to gather and refine the ryetalyn.
The first batch contains irillium, an impurity that makes it useless, and a second must be prepared while the landing party is drawn into Flint's secrets: his immense age, and the android Rayna he has built as a companion. McCoy finally obtains pure ryetalyn and the epidemic is stopped.