Jethrik is a blueish mineral that can be used as fuel. Though harmless in its natural form, a handful of Jethrik can power an entire starship. The scarcity of Jethrik makes it all the more valuable.
Knowing that the Graff Vynda-K needed Jethrik for his war efforts, two confidence tricksters, Garron and Unstoffe came to the the primitive world of Ribos to swindle him. Garron offered to sell the planet to Garron and Unstoffe to pose as a native.
The pair hoped to convince the Graff Vynda-K to buy Ribos by convincing him that the planet had abundant mineral deposits of Jehrik, and that the natives, having no advanced technology, had no clue as to its value. To this end, Garron had Unstoffe pose as a native and tell a story about the abudanced of "scryngestone" (a made-up name for Jethik) on the planet.
To this end, they had to smuggle a large chunk of Jethrik into a Ribos museum so that the Graff could see it. In reality, the chunk of Jethrik was the first segment of the Key to Time took the form of a large chunk of jethrik on the planet Ribos. (DW: The Ribos Operation)