"I should open it first..." Arnold mumbled, thinking about the material he would get from opening the small box, "I'll take anything apart from the celestial copper. Getting that would be useless."
Talos had already mentioned about the properties of the listed materials. So Arnold had his mindset on the materials that he wanted to get out of the material box.
He thought it would be good if he got celestial gold or silver as they were the essential materials for making God slaying weapons like Brahmastra, something which was much needed by him as the last time he met a 'lesser god' she tried her best to kill him and was almost successful in doing so. He couldn't help but think if he had a god slaying weapon back then, [The Guide Between the Worlds] wouldn't have been to bully him so easily.