Warning: spoilers ahead for The Rings of Power episodes 1 & 2What's the mysterious glow emanating from Durin III's treasure chest in Robert Aramayo's Elrond his blueprints for a great forge, but both elves know such a feat cannot be accomplished within the desired time frame. If only Elrond had a good friend with thousands of followers and expert architectural skills... Reuniting with Durin IV twenty years after their last meeting, Elrond receives a frosty welcome when his dwarf buddy takes offense to two decades without a visit. Their differences are (eventually) settled over dinner, and Durin IV brings Elrond's forge-building proposal to his father.
This is where things get strange in The Rings of Power episode 2. Durin III asks his son whether Elrond showed any sign of being suspicious. Durin IV answers no, but Khazad-dûm's king is unconvinced, believing the elf's arrival is too great a "coincidence" to ignore. The Durins then produce a chest, the contents of which glow silver in the dwarves' faces, but deliberately aren't shown to the viewer. Whatever lies inside this box, Durin III didn't want Elrond knowing the dwarves possessed it...
Assuming Durin IV hasn't stolen Marcellus Wallace's briefcase from The Lord of the Rings, so it makes sense that dwarves began harvesting mithril during The Rings of Power's Second Age era. Out of fear they'd want it for themselves, any dwarf would loathe to let those pesky Elves learn about such a promising discovery, and this explains Durin III's suspicion over Elrond.
Do The Dwarves Possess A Silmaril In The Rings Of Power?!
Another (considerably less likely) possibility is that Durin III's box in The Rings of Power episode 2 contains a Silmaril. Forged by Fëanor several millennia before The Rings of Power's time, the three Silmarils are supposed to be lost - one in the sky, one in the sea, and one under the earth. Maybe the dwarves of Khazad-dûm happened across the Silmaril lost in earth during their mining efforts. This would explain the bright glow, why the item is kept by the king in a special box , and why Celebrimbor and Elrond drop a ton of Silmaril history exposition earlier in the episode. The Elves and Dwarves fought over a Silmaril in the past, and that incident caused bad blood that never really goes away. If the Durins really had dug a Silmaril out of Moria, they'd absolutely want to keep that secret from any elves. Were the news to get out, Fëanor's descendants would be knocking down the door of Moria before you can say "mellon."
Don't go reaching for your copy of J.R.R. Tolkien's The Silmarillion just yet, however - mithril is still the far more likely outcome. Dwarves finding a Silmaril in Second Age Khazad-dûm would represent a huge change to existing mythology, and throw the entire landscape of Middle-earth into chaos. Furthermore, you'd imagine a Silmaril would glow a lot brighter than whatever lies in Durin III's box. The Rings of Power's unseen gem gently shines onto the father and son's faces like a phone screen; a Silmaril would be closer to a car headlamp bathing the dwarves' corneas in brilliant light.
The Lord of the Rings: The Rings of Power continues Thursday/Friday on Prime Video.