brass
鉐 centers on brass, a metal alloy. The meaning is specific to this metallic substance.
鉐 combines 金 (metal) with 石 (stone), likely indicating a metal with stone-like qualities or a metal extracted from stone. The exact historical development is uncertain.
Metal 金 and stone 石 together make brass, a metal that can feel as solid as stone.
For セキ, imagine a brass key made of stone: seki (セキ) sounds like 'secky', a made-up word for a stone key. The brass key is as heavy as stone.
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