A Silmaril is one of the three fictional star-jewels created by Fëanor in Tolkien’s mythology.
Fëanor was a brilliant but proud Ñoldorin Elf. He captured the light of the Two Trees of Valinor inside three jewels called the Silmarils. Their radiance made them the greatest treasures of the First Age, and their creation became one of the defining events of Tolkien’s mythology.
Morgoth stole the Silmarils after destroying the Two Trees and carried them to Middle-earth. Fëanor and his sons swore an oath to recover them, an act that led to rebellion, exile, and generations of war. The central stories of The Silmarillion are therefore shaped by the jewels’ beauty and by the destructive oath surrounding them.
The singular is Silmaril; the plural is Silmarils. They are not the same as the Arkenstone in The Hobbit, the Palantíri used for seeing, or the Elessar associated with Aragorn.