Unfinished Tales, part two: The Second Age - chap. III The Line of Elros: Kings of Numenor
"XV Tar-Telemmaitë
He was born in the year 2136, and ruled for 140 years, until his death in 2526. Hereafter the Kings ruled in name from the death of their father to their own death, though the actual power passed often to their sons or counsellors; and the days the descendants of Elros waned under the Shadow. This King was so called because of his love of silver, and he bade his servants to seek ever for mithril."
Unfinished Tales, part three: The Third Age - chap. I The Disaster of the Gladden Fields
note 31 "For that metal was found in Númenor. [Author's note.] – In "The Line of Elros" (p.232) Tar-Telemmaitë, the fifteenth Ruler of Númenor, is said to have been called so (i.e. "silver-handed") because of his love of silver, "and he bade his servants to seek ever for mithril." But Gandalf said that mithril was found in Moria "alone in the world" (The Fellowship of the Ring II 4)".
NB: pour l'anecdote, selon HoME VI (chapitre 25 notes 34 et 35) et Home VII (chapitre 9), le nom Mithril n'est apparu qu'en 1966 pour la troisième édition du Hobbit>/i>. Auparavant, il était seulement question, dans The Hobbit de "silvered steel". Quant aux brouillons des premières versions du SdA, Tolkien utilisa d'abord le terme 'Erceleb', puis 'Ithil' et 'Thilevril' avant de différencier l'Ithildin et le Mithril.