Jethrik was a blueish mineral that was a vital component for certain types of warp drive engines. Though harmless in its natural form, a handful of jethrik could power an entire starship. The scarcity of jethrik made it valuable.
Knowing that the Graff Vynda-K needed jethrik for his war efforts, two confidence tricksters, Garron and Unstoffe, came to the primitive planet of Ribos to swindle him. The pair hoped to convince the Graff Vynda-K to buy Ribos by convincing him that the planet had abundant mineral deposits of jethrik and that the natives, with no advanced technology, had no clue to its value. To this end, Garron had Unstoffe pose as a native and tell a story about the abundance of "scryngestone" (a made-up, native name for jethrik) on the planet.
They had smuggled a large quantity of jethrik into the city of Shur so that the Graff could see it. Unbeknownst to them, the jethrik was the first segment of the Key to Time. (TV: The Ribos Operation)