Durgolium

A white and ancient alloy forged to contain objects from other planes. Originally developed by the demigod Dorgren over 3000 years ago when Azema was present in the world.

It's said Durgus was able to forge it in Nalos during Queen Gimnax's reign from an ore found in the Serrated Mountains.

“There are durgolium vaults hidden all over Nalos. Each holds an object of great power. We cannot let them fall into the wrong hands. By the king’s orders, we must gather as many as we can, and with haste.” - Tarik Nowlin, Battle at Blacklake (RPA-ADV1) 

Rings out a remarkably pleasant tone when struck.

Durgolium is a near-indestructible substance, first seen as a chest guarded by the Ogre Tog;

Between his angry cries, he smashes the metal chest with such force that it shakes the ground around you. But the chest only rings out a remarkably pleasant tone. It remains unbroken, undented, and firmly in place. - Battle at Blacklake (RPA-ADV1)

Upon opening the chest, Tarik gives you a sack made of the same metal to extract and house the prize within. Within the chest is a jar of Gray Matter. You are then given other chainmail pouches like it to secure the other interplanar objects scattered across Ulos;

Tarik pulls several empty durgolium chainmail sacks from his pack and hands them to you. - Battle at Blacklake (RPA-ADV1)

In Jolev, Gimlix reveals that durgolium is an ancient metal;

“Sadly, no one knows how to make durgolium anymore. The closest thing we have is Elvish silver, but even the Elves can only produce crude imitations of work like this.” - Gimlix

A durgolium trapdoor and a durgolium bell in the Ancient Chapel reveal that this mystic material was used to craft objects of great importance in Nalos history. Within the chamber of the trapdoor is the Jade Gauntlet.

The desk of Bertram Hoogna is made of durgolium. Inside resides the Onyx Cube.

The durgolium writing desk in the library is missing its inkwell and the marble head of Mr. Bertram Hoogna. - Outbreak in Undercity (RPA-ADV3)

A door of durgolium can be found in the Temple of Ulos, protecting the Eye of Ulos, as well as a seal in the Great Hall of Sabek Palace.

A durgolium seal decorated with ancient symbols graces the center of the hall’s floor. - Eye of Ulos (RPA-ADV4)

A record of Azema taken by Finneaus the Kobold of Urkensheim cites Dorgren as the crafter of “Dorgren's alloy”;

“My time is near. Go now and collect the six artifacts from my gate. Bury them in chambers built of Dorgren’s alloy. Keep them safe for three thousand years. Then open my gate once more, so that I may return and bless the faithful. I ask only that you remain strong and united as one people, as my people, until my return.” - (RPA-ADVF)

When traveling to Nebblis, Plane of Fire, the Red Pearl is revealed to be the seed of a tree, the bark of which is raw durgolium;

It seems you have planted a seed in this plane, and it quickly takes root. A white-silver tree sprouts rapidly from the red sand. It grows thick and spreads its branches overhead while its gnarled roots crack the stone ground, creating fissures of sand. Into these fissures, the tree drops its acorns, and new sprouts spring up all around you. Before long, you stand in a forest of white-silver trees. Soon, the whole plane will be filled with them. You inspect their bark more closely and find that the trees are made of unrefined durgolium. - Gate of Brizoor Waal (RPA-ADVF)