high quality iron
鍇 centers on iron of superior quality, specifically high-grade iron. The meaning is narrow and concrete, without abstract extensions.
鍇 combines 金 (metal) with 皆, which likely contributes the sound and may suggest the idea of all or complete, perhaps indicating fully refined iron.
The left side is metal 金, and the right side 皆 looks like a container with a lid. Picture a special crucible holding the finest iron, all of it high quality.
For カイ, imagine a blacksmith shouting 'Kai!' as he pulls a gleaming bar of high-quality iron from the forge: Kai -> カイ.