The Silmarillion

Then Manwë granted him pardon; but the Valar would not yet suffer him to depart beyond their sight and vigilance, and he was constrained to dwell within the gates of Valmar. But fair-seeming were all the words and deeds of Melkor in that time, and both the Valar and the Eldar had profit from his aid and counsel, if they sought it; and therefore in a while he was given leave to go freely about the land, and it seemed to Manwë that the evil of Melkor was cured. For Manwë was free from evil and could not comprehend it, and he knew that in the beginning, in the thought of Ilúvatar, Melkor had been even as he; and he saw not to the depths of Melkor’s heart, and did not perceive that all love had departed from him for ever. But Ulmo was not deceived, and Tulkas clenched his hands whenever he saw Melkor his foe go by; for if Tulkas is slow to wrath he is slow also to forget. But they obeyed the judgement of Manwë; for those who will defend authority against rebellion must not themselves rebel.

Now in his heart Melkor most hated the Eldar, both because they were fair and joyful and because in them he saw the reason for the arising of the Valar, and his own downfall. Therefore all the more did he feign love for them and seek their friendship, and he offered them the service of his lore and labour in any great deed that they would do. The Vanyar indeed held him in suspicion, for they dwelt in the light of the Trees and were content; and to the Teleri he gave small heed, thinking them of little worth, tools too weak for his designs. But the Noldor took delight in the hidden knowledge that he could reveal to them; and some hearkened to words that it would have been better for them never to have heard. Melkor indeed declared afterwards that Fëanor had learned much art from him in secret, and had been instructed by him in the greatest of all his works; but he lied in his lust and his envy, for none of the Eldalië ever hated Melkor more than Fëanor son of Finwë, who first named him Morgoth; and snared though he was in the webs of Melkor's malice against the Valar he held no converse with him and took no counsel from him. For Fëanor was driven by the fire of his own heart only, working ever swiftly and alone; and he asked the aid and sought the counsel of none that dwelt in Aman, great or small, save only and for a little while of Nerdanel the wise, his wife.

Chapter 7 Of the Silmarils and the Unrest of the Noldor

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.

As three great Jewels they were in form. But not until the End, when Fëanor shall return who perished ere the Sun was made, and sits now in the Halls of Awaiting and comes no more among his kin; not until the Sun passes and the Moon falls, shall it be known of what substance they were made. 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 Ilúvatar: 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 hands 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 that he himself had made.