§ Of the Silmarils and the Unrest of the Noldor The Silmarillion

In that time were made those things that afterwards were most renowned of all the works of the Elves. For Fëanor, being come to his full might, was filled with a new thought, or it may be that some shadow of foreknowledge came to him of the doom that drew near; and he pondered how the light of the Trees, the glory of the Blessed Realm, might be preserved imperishable. Then he began a long and secret labour, and he summoned all his lore, and his power, and his subtle skill; and at the end of all he made the Silmarils.