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At the end, we collected roughly 56 pieces of high-quality spirit stones of various elements, some of them of the rare Darkness, Poison, Time, and Space Elements! I'll make sure to use those for my familiar evolutions. I think Glutton needed Space Attribute Spirit Stones for his big evolution, as many as I can procure.
Aside from that, we found large piles of gold, silver, mithril, and a green crystal that the system called something like "Uranium" or something. It emanated a strong, pressuring aura. But it didn't affect us negatively when we touched it, we were all strong enough to easily withstand this level of poisoning, but it might be dangerous to expose people with Physiques below Tier 3 to this.