[That compliment in fact means everything. Izuku had been perfectly ready to believe that he had messed up, that the geode was ruined now, but having someone else look at the outcome and consider it good? And say so, with no quantifiers or buts? It helps. It doesn't completely counter those negative thoughts - he's still convinced he bungled it - but it's enough to make it feel like it at least wasn't the total disaster he'd set himself up to believe it was.]

I'm not actually sure? I mean, I'm used to seeing them polished up to be decorations, but I don't know if that's what they do here.