Was Mithril Discovered after the fall of Belegost and Nogrod?

[Timmity Took]

Mithril was discovered in Khazad Dûm, but was this after the fall of Belegost and Nogrod?

[Stormcrow]

I don't think a date for the discovery of mithril is ever given. Khazad-dum was way off at the edge of Eregion, far from the notice of elves in Beleriand. Who knows how long the Longbeards of Khazad-dum had been mining it before the outside world heard of it?

If I had to take a guess, I'd guess that the dwarves of Belegost and Nogrod did not know about mithril, otherwise the Nauglamir would undoubtedly have been made of it. So either the Longbeards had already found it and didn't tell anyone who brought that information back to Beleriand, or they hadn't found it yet.

They have to have found it by the early Second Age, because the Noldor founded Eregion to be near Khazad-dum and its mithril. Unfinished Tales suggests mithril was discovered in Numenor, too. So early Second Age sounds about right to me.

[Otaku-sempai]

The Dwarves of Khazad-dûm might have discovered mithril during the First Age, but kept the secret of its origins to themselves. The Dwarves of Belegost and Nogrod might have known of its existence, but could have been led to assume that it was be an alloy developed by their cousins in the Misty Mountains and not a metal unto itself. In either case, the Dwarves of the Blue Mountains don't seem to have had access to mithril before they relocated to Moria after abandoning their ruined cities early in the Second Age.