"Weel done, Cutty-sark!" -Mithril, Moria and Valinorean ship-building

[Formendacil]

If memory serves, there's a footnote somewhere in Unfinished Tales that provides the basis for the speculation about mithril being found in Valinor and Númenor.

However, I'd like to address the Might:

Originally Posted by the Might

No, clearly it is implied in the world at that time.

Actually...

In Tolkien's earlier writings, especially, "the world" quite often does NOT inclusively refer to Valinor. This was particularly the case in the The Book of Lost Tales, when the conception of the Straight Road and the the Round World after the Akallabêth had not yet been conceived. Tolkien several times refers to "the world" as "everywhere but Aman"--or what afterwards came to be named Aman.

I'm not sure how prevalent this usage of "the world" becomes after the Book of Lost Tales, but I think it would be unwise to write off the possibility of Aman having mithril simply on the basis of it having been "in the world".