Charcoal
With the two halves of the eggshell in his arms, Eisen made his way toward the room especially created for him to work in, as the walls were enchanted both in a way to allow maximum usage of Eisen's 'Crafting Space' skill, while also making sure the heat didn't damage the walls in any way.
Here, as the space was quite small and he couldn't place all of his workstations inside, Eisen used his staff to create the foundry where he would extract the metal from the petrified eggshell. He didn't exactly know which metals were inside it, but he was sure that they were mostly metals with a relatively low reaction, albeit not noble metals, which was quite perfect for extraction.