The Silmarils (Quenya pl. Silmarilli, radiance of pure light) are three fictional brilliant jewels composed of the unmarred light of the Two Trees in J. R. R. Tolkien's legendarium. The Silmarils were made out of the crystalline substance silima by Fëanor, a Noldorin Elf, in Valinor during the Years of the Trees. The Silmarils play a central role in Tolkien's book The Silmarillion, which tells of the creation of Eä (the Universe) and the beginning of Elves, Men, and Dwarves.
How Fëanor, admittedly the greatest of the Noldor, was able to create these objects is not fully explained. Even the Valar, including Aulë, master of craftsmanship, could not copy them. In fact, even Fëanor may not have been able to copy them as part of his essence went into their making. Their worth, in Tolkien's universe, was very great, even to the Valar, as they were unique and irreplaceable. The Silmarils themselves are said to produce their own light, which comes from the Two Trees, but also to reflect the light of any other lights that come near them.
Fëanor, son of Finwë, created the Silmarils—"the most renowned of all the works of the Elves"—from the light of the Two Trees. The Silmarils were hallowed by Varda, so that they would burn the hands of any evil creature or mortal who touched them without justly deserving possession.
Together with Ungoliant, the rebellious Vala Melkor destroyed the Two Trees. (Later, at the healing effort of the Valar, one of the trees bore a silver flower, and the other bore a golden fruit before their death. They were sent to the sky, to illuminate Middle-Earth against Melkor. But neither sphere radiated the original light of the trees, that was free of Ungoliant's poison.) Silmarils then contained all the remaining unmarred light of the Two Trees. Therefore, the Valar entreated Fëanor to give them up so they could restore the Trees, but he refused. Then news came that Melkor had killed Fëanor's father Finwë, the High King of the Noldor, and stolen the Silmarils. After this deed, Melkor fled from Valinor to his fortress Angband in the north of Middle-earth. Thereafter he wore the Silmarils in his iron crown.