The fictional artifacts Tolkien describes as Fëanor’s greatest creation are the Silmarils.
The Silmarils are three jewels made by Fëanor, the most gifted craftsman among the Noldorin Elves. They contain the light of the Two Trees of Valinor, Telperion and Laurelin, which existed before the Sun and Moon. Their radiance and sacred origin make them unique in Tolkien’s mythology.
The jewels are stolen by Morgoth, the first Dark Lord, who sets them in his crown. Fëanor and his sons swear a terrible oath to recover them, and that oath drives much of the tragedy narrated in The Silmarillion. The wars over the jewels affect Elves, Men, and other peoples across the First Age.
The Silmarils are not simply valuable treasure. Their story concerns creative pride, possessiveness, exile, oath-breaking, and the destructive consequences of trying to reclaim something that has been lost. Their fate also links the mythology of Valinor with the history of Middle-earth.