A rare mineral essential to the life-support systems of planetoid colonies, found in abundance on Capella IV and sought by both the Federation and the Klingon Empire.
Topaline is the prize in the Original Series episode Friday's Child. Certain Federation colonies cannot keep their artificial environments running without it, and Capella IV is one of the few rich sources.
Kirk, Spock and McCoy arrive to negotiate mining rights with the Capellans, a warrior people among whom McCoy once served. They find the Klingon agent Kras already there, offering a rival agreement and encouraging a coup against the Teer, Akaar.
Akaar is killed and the landing party flees into the hills with his pregnant widow Eleen. After Kras is slain and order returns, Eleen, acting as regent for her newborn son Leonard James Akaar, signs the topaline treaty with the Federation.