The powerful jewels sought by Fëanor’s sons in J.R.R. Tolkien’s First Age stories are the Silmarils. Fëanor creates three jewels containing the light of the Two Trees of Valinor.
The jewels become the central objects of the tragic history told in the Quenta Silmarillion. Morgoth steals them after destroying the Two Trees, takes them to Middle-earth, and sets them in his iron crown.
Fëanor and his seven sons swear an oath to recover the Silmarils, regardless of who possesses them. That oath drives conflicts among Elves and contributes to wars, betrayals, and the downfall of important kingdoms.
The Silmarils are not the same as the Arkenstone. The Arkenstone is a single Dwarven heirloom found in Erebor, while the Silmarils are three ancient jewels associated with the First Age and the light of Valinor.