They consist of an unknown crystal-like substance and are filled with the light of the Two Trees, Telperion and Laurelin, called The Living Light.
At first displaying them readily and allowing them to be hallowed by the Valar, Feanor eventually began to love the Silmarils with undue greed and withheld them from anyone except his father and sons. They attracted the attention and greed of Morgoth as well, who eventually broke into Feanor's northern fortress of Formenos, slew Finwe, and stole the Silmarils with the help of the spider demon Ungoliant. Feanor and his sons swore a terrible Oath to regain them. Back in Angband, Morgoth put the Silmarils into his Iron Crown where they remained for several centuries, until Doriathrim King Thingol demanded one Silmaril as bride-pride of Lúthien his daughter. One of the jewels was subsequently won by Beren and Lúthien, although it came at a price, brought about the death of Thingol and the first Fall of Doriath at the hands of the Dwarves. It also reawakened the Oath of the Feanorians when it passed into the hands of Lúthien's and Beren's son Dior and caused the attack of the Sons of Feanor on Doriath. The Silmaril escaped with Dior's daughter Elwing to the Havens of Sirion, later also site of the Third Kinslaying. Elwing threw herself into the sea with the jewel to evade capture, and in a seabird's shape found her husband Eärendil the mariner. Together they passed into the Blessed Realm and sued for pardon for the Elves of Middle-earth. Set on Eärendil's brow, the Silmaril now sails the skies as the star of morning and evening, called Gil-Estel (Star of Hope) by the Elves.
The remaining Silmarils were regained in the War of Wrath and left in Eönwe's keeping. However, Maedhros and Maglor stole them and fled, subsequently realising they had lost their claim on the jewels. Maedhros cast himself into a fiery chasm with his jewel, Maglor cast his Silmaril into the sea, thus fulfilling the prophecy that the fate of Earth, Sea, and Sky lay locked within them.
It is said that the Silmarils will be unlocked in or after the Dagor Dagorath, various sources attribute this deed either to Feanor or Maedhros.