The Silmarils are the three jewels that give J.R.R. Tolkien’s The Silmarillion its title.
The craftsman Fëanor creates the Silmarils, enclosing the light of the Two Trees of Valinor within them. They are regarded as the greatest works of Elven craftsmanship. Their beauty and sacred light make them objects of desire for both allies and enemies.
Morgoth steals the jewels and sets them in his crown. Fëanor and his sons swear an oath to recover them, an oath that drives much of the tragic history of the First Age. The wars and disasters surrounding the jewels include the Kinslayings, the exile of the Ñoldor, and the ruin of several Elven kingdoms.
The three Silmarils ultimately have different fates: one is taken into the sky, one falls into the sea, and one into the earth. The Arkenstone in The Hobbit is another famous jewel, but it is not one of the Silmarils.