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From the BoLT1; "The Chaining of Melko":

"Behold, Aulë now gathered six metals, copper, silver, tin, lead, iron, and gold, and taking a portion of each made with his magic a seventh which he named therefore tilkal,* and this had all the properties of the six and many of its own. Its colour was bright green or red in varying lights and it could not be broken, and Aulë alone could forge it. Thereafter he forged a mighty chain, making it of all seven metals welded with spells to a substance of uttermost hardness and brightness and smoothness, but of tilkal he had not sufficient to add more than a little to each link. Nonetheless he made two manacles of tilkal only and four fetters likewise. Now the chain was named Angaino, the oppressor, and the manacles Vorotemnar that bind for ever, but the fetters Ilterendi for they might not be filed or cleft."