The Lord of the Rings: The Rings of Power is still a great deal of work to learn about Middle-earth's history, or at least its version of it. Perhaps the most interesting part has to do with mithril, a legendary elf, and the Misty Mountains itself.
This article contains spoilers for The Lord of the Rings: The Rings of Power episode 5, Partings.
During the fifth episode of the season, High King Gil-Galad and Elrond discussed the knowledge they learned from Durin. After Elrond refused to reveal the information he learned from Durin following his oath last episode, Gil-Galad asked him to recount The Song of the Roots of Hithaeglir.
Elrond describes the poem as depicting a pure-hearted elf warrior and a balrog of Moria over a beautiful tree at the highest of the Misty Mountains. According to the legend the tree contained one of the lost Silmarils, a set of gems that served as the outcome of the conflict with Morgoth.
The elf poured his light into the tree, while the balrog attempted to infiltrate it with its own evil and darkness. In the midst of this struggle, the tree was struck by lightning, and its essence penetrated deep into the Misty Mountains below and became mithril.
No one knew for sure that mithril was real until Durin and his people discovered it beneath their halls in Khazad-dum, but Gil-Galad seems to have always suspected it and was holding out hope that it might be mined.
Why is Mithril so important to the Elves? Perhaps because to a different tree. This tree, Gil-Galad suggests, is slowly fading in power, losing its power over the world around them. Only by infusing mithril, which contains the power of the Silmarils, that Gil-Galad's homeland, can the elves be whole again.
With all of this lore, it's probably worth noting that most of the above is an adaptation of the show rather than Tolkiens original work. Mithril was never discovered in other mines than Moria, such as those in Numenor, and it is likely to be used again in the future. However, we do know that mithril has a significant role to play as the Rings of Power continues.