This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
硡 combines 石 (stone) with 宏, which may contribute sound. The exact historical development is uncertain.
The stone radical 石 suggests something related to rock or mineral, but without a known meaning, the kanji is best remembered as a stone paired with the sound component 宏.
For コウ, imagine a stone echoing with a deep 'koh' sound when struck: koh -> コウ.
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