In J.R.R. Tolkien’s mythology, the powerful jewels stolen by Morgoth are the Silmarils.
The Silmarils were three radiant jewels made by Fëanor, the greatest craftsman among the Elves of Valinor. They captured the light of the Two Trees of Valinor, Telperion and Laurelin. Their beauty and sacred light made them objects of extraordinary desire.
Morgoth, formerly called Melkor, stole the jewels during his assault on Valinor and carried them to Middle-earth. Fëanor and his sons swore an oath to recover them, an act that caused generations of violence and tragedy among Elves, Men, and other peoples.
The Silmarils are the central objects of Tolkien’s legendarium’s First Age stories, especially The Silmarillion. They are not the same as the Arkenstone, which is a famous jewel in The Hobbit, or the One Ring, which Sauron forged much later. Tolkien’s singular form is Silmaril; the plural is Silmarils.