This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
礚 combines 石 (stone) with 蓋, which likely contributes the sound. The exact historical development is uncertain.
The stone radical 石 beside 蓋 suggests something related to stone or rocks, but the exact meaning is unclear.
For カイ, imagine a stone lid (蓋) that opens with a creaky 'kai' sound: kai -> カイ.
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