The three jewels stolen by Morgoth in J.R.R. Tolkien’s legendarium were called the Silmarils.
Fëanor, the Ñoldorin Elf, created the Silmarils during the Years of the Trees. They held the light of the Two Trees of Valinor, making them the most treasured objects in the world of the Elves.
After Morgoth destroyed the Two Trees and stole the jewels, Fëanor and his sons swore an oath to recover them. That oath drove the Ñoldor into exile and caused generations of conflict, including wars among Elves, Men and Morgoth’s forces.
The Silmarils are not the same as the Rings of Power, the palantíri or the Arkenstone. Their story belongs primarily to the First Age and forms the central thread of The Silmarillion’s title and history.