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.
As three great jewels they were in form. . . . Like the crystal of diamonds it appeared and yet was more strong than adamant, so that no violence could mar it or break it within the kingdom of Arda. Yet that crystal was to the Silmarils but as is the body to the Children of Illúvatar: the house of its inner fire, that is within it and yet in all parts of it, and is its life. And the inner fire of the Silmarils Fëanor made of the blended light of the Trees of Valinor, which lives in them yet, though the Trees have long withered and shine no more. Therefore even in the darkness of the deepest treasury the Silmarils of their own radiance shone like the stars of Varda; and yet, as were they indeed living things, they rejoiced in light and received it and gave it back in hues more marvellous than before.
All who dwelt in Aman were filled with wonder and delight at the work of Fëanor. And Varda hallowed the Silmarils, so that thereafter no mortal flesh, nor hand unclean, nor anything of evil will might touch them, but it was scorched and withered; and Mandos foretold that the fates of Arda, earth, sea, and air, lay locked within them. The heart of Fëanor was fast bound to these things he himself had made.
Then Melkor lusted for the Silmarils, and the very memory of their radiance was a gnawing fire in his heart. From that time forth, inflamed by this desire, he sought ever more eagerly how he should destroy Fëanor and end the friendship of the Valar and the Elves; but he dissembled his purpose with cunning and nothing of his malice could yet be seen in the semblance that he wore. Long was he at work, and slow at first and barren was his labour. But he that sows lies in the end shall not lack of harvest, and soon he may rest from toil indeed while others reap and sow in his stead. Ever Melkor found some ears that would heed him, and some tongues that would enlarge what they had heard; and his lies passed from friend to friend, as secrets of which the knowledge proves the teller wise. Bitterly did the Noldor atone for the folly of their open ears in the days that followed after.
Thus ere the Valar were aware, the peace of Valinor was poisoned. The Noldor began to murmur against them, and many became filled with pride, forgetting how much of what they had and knew came to them in gift from the Valar. Fiercest burned the new heart of Fëanor; and Melkor laughed in his secrecy, for to that mark his lies had been addressed, hating Fëanor above all, and lusting ever for the Silmarils.
This narrative leads to one of the most compelling in the Silmarillion, the story of how Fëanor denied the Silmarils to the Valar to restore the Trees of Valinor. He says, “This thing I will not do of free will. But if the Valar will constrain me, then I shall know indeed that Melkor is of their kindred.” The idolatry is complete. And yet, what is tragic is that the Silmarils were not even his to give at this point since they had already been stolen and his father killed, facts he would find out momentarily. Then in his grief and rage he curses not only Melkor, but also the summons of Manwë. The narrator comments, “The Silmarils had passed away, and all one it may seem whether Fëanor had said yea or nay to Yavanna; yet had he said yea at the first, before the tidings came from Formenos, it may be that his after deeds would have been other than they were. But now the doom of the Noldor drew near.”