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Episode 4 of The Watch Hugas OnlineLord of the Rings: The Rings of Powerfinally reveals what the dwarves of Khazad-dûm have been mining in secret: mithril!

A precious metal invented by J.R.R. Tolkien for his novels, mithril is both incredibly light and impossibly strong. Plus, its beauty never fades. If it sounds familiar, that's because mithril plays a key role in the Third Age of Middle-earth. Thorin gifts Bilbo a shirt of mithril mail in The Hobbit, which Bilbo then gifts in turn to Frodo in The Fellowship of the Ring. That shirt saves Frodo's life during combat; without mithril, he would have been a total goner. However, that's all Third Age business. The Rings of Power takes place during the Second Age of Middle-earth, so we get to see how the world reacts to the first discovery of such a precious ore.

How does mithril fit into The Rings of Power?

While visiting Khazad-dûm, Elrond (Robert Aramayo) finds that Prince Durin (Owain Arthur) and the dwarves have been covertly mining a newly discovered substance. In the dwarven tongue, the name for the metal means "grey glitter." When translated to Sindarin, you get the Elven word "mithril."

Durin tells Elrond that Princess Disa (Sophia Nomvete) was the first to detect mithril. Upon learning of its lightness and durability, Durin reckons that it could be the start of a new era for the dwarves. Unfortunately, mithril proves dangerous to mine, and his father would rather exercise caution than launch full-speed ahead into a mining endeavor.

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If you're familiar with Middle-earth history, you'll know that the mining proceeds anyway, lasting all the way into the Third Age. The dwarves delve too greedily and too deep in their search for mithril, awakening a terrible Balrog and forcing them to abandon Khazad-dûm for good. This Balrog is the very same being that the Fellowship of the Ring encounters in the Mines of Moria.

Since much of The Rings of Power takes liberties with Tolkien's work, it's safe to say that this origin story for mithril is a new invention for the show. However, Tolkien didn't write much about the finding of mithril at all. In Appendix B of The Lord of the Rings, he mentions that elves moved to Eregion when they learned of the dwarves' discovery of mithril in Moria, but that's about it.

The Rings of Power has taken some liberties with this information, as Eregion is already established by the time the dwarves have found mithril. Still, this change opens up some interesting narrative possibilities for the future. Elrond swore to Durin that he would keep mithril a secret — will his loyalty to the elves be tested against his friendship to Durin? Will mithril be another point of tension between the dwarves and the elves?

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Most pressingly: What does the emphasis on mithril mean for the actual forging of the Rings of Power? Based on the name of the show, we know the creation of the Rings of Power (including the One Ring) will play a large part in this series. Perhaps mithril will figure more largely into the actual forging of the rings.

In Tolkien's work, only one of the rings — Nenya, the ring worn by Galadriel – is explicitly described as being made of mithril. However, given the lack of detail about some of the other rings and The Rings of Power's tendency to diverge from what's on the page, I wouldn't be surprised if more of the Rings of Power got the mithril treatment moving forward.

The Lord of the Rings: The Rings of Power is now streaming on Prime Video. New episodes release every Friday at 12 a.m. ET.