Demons are immune.

For humans, it strengthens them, but chips away at their sanity. For witches, it restricts their power. For demons, it doesn't affect them at all. It's only an ordinary gem for them. Beastmen, on the other hand lose their physical prowess due to this gem, and hence cursed ore is popularly used as slave collars for beastmen. As for monsters, no one knows yet. They fled before cursed ore was discovered in this world. You are immune to this cursed gem just like the demons."

Harry carefully listened to him and then raised a doubt. "In this world? That means cursed ore was discovered in many worlds before right? Then how is it's effect towards monsters not known?"