The Silmarils were the fictional star-jewels sought by Morgoth in J.R.R. Tolkien’s The Silmarillion. The three jewels were made by the elf Fëanor and contained the light of the Two Trees of Valinor.
After Morgoth stole the Silmarils, Fëanor and his sons swore an oath to recover them. Their pursuit drives much of the First Age history, including wars, betrayals, and the destruction of several kingdoms. The jewels are not merely valuable objects: their light and sacred origin make them central to the mythology’s conflict between creation, possession, and loss.
The Silmarils should not be confused with the Arkenstone, which appears in The Hobbit, or the One Ring, which dominates The Lord of the Rings. The plural answer is important because Tolkien’s legendarium contains three Silmarils, not one.