The crystals said to channel the power of the starship Enterprise in the earliest Original Series episodes, mined on Rigel XII and processed on Delta Vega, before the writers settled on the fictional dilithium.
Lithium crystals are the material named as essential to the Enterprise's engines in the first episodes of The Original Series. Without sound crystals the ship's power circuits burn out, and replacements are rare enough to be worth more than gems.
In Where No Man Has Gone Before, the damaged ship limps to the automated lithium cracking station on Delta Vega for repairs, and Kirk plans to leave Gary Mitchell there. In Mudd's Women, set in 2266, the ship's crystals fail one by one during the pursuit of Harry Mudd, and Kirk must bargain with the three lonely miners of Rigel XII, led by Ben Childress, who want Mudd's companions as wives in exchange.
Soon afterwards the series began to speak of dilithium instead, an invented substance that freed the writers from the properties of a real element. Lithium crystals are not mentioned again.