Even if the common sense of that world states nobody should be able to use a 10-star skill orb, it still wouldn't be left lying in the mine warehouse, to collect dust. It would definitely be in the royal treasury, locked away. Because the king wouldn't take the chance and accept the risk that someone out there might be able to use it, following previously unknown methods. A 6-star orb got a bloody duke to visit the stinky mine in person. It's such a big deal. 10-star would be an even bigger thing, despite being currently unusable. But since it exists, people wouldn't be so quick to offhandedly dismiss it as trash. But then again, maybe they would. This is Japanese manga and these stories are full of people labeling many skills as trash.