The jewels central to the title of J.R.R. Tolkien’s The Silmarillion are the Silmarils. They are three radiant jewels created by Fëanor, the gifted Ñoldorin Elf, during the Years of the Trees.
The Silmarils contain the light of the Two Trees of Valinor, Telperion and Laurelin. Their beauty and sacred light make them objects of desire, but Fëanor’s possessiveness and the oath sworn by him and his sons drive much of the tragedy of the First Age.
Morgoth steals the jewels after destroying the Two Trees and carries them to Middle-earth. The wars fought to recover them form the core of the Quenta Silmarillion, one of the book’s major sections.
The Silmarils are not the same as the One Ring, the Arkenstone, or the green jewel called the Elessar. Those objects belong to different stories and have different origins, even though all are important artifacts in Tolkien’s legendarium.