A Silmaril is one of the three fictional star-shaped jewels made by Fëanor in J.R.R. Tolkien’s legendarium.
The Silmarils contained the light of the Two Trees of Valinor, Telperion and Laurelin. Fëanor, the greatest craftsman among the Noldor, created them before the First Age. Their beauty and sacred light made them objects of desire, but Fëanor’s possessiveness and oath to recover them drove much of the tragedy of the First Age.
Morgoth stole the jewels and set them in his iron crown. The resulting wars involved Elves, Men, and other peoples, and the three Silmarils eventually came to different fates: one in the sky, one in the sea, and one in the earth. The Silmarils are not the same as the Arkenstone, which is a separate jewel associated with Erebor in The Hobbit.