This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
砎 combines 石 (stone) with 介, which likely contributes sound. The exact historical development is uncertain.
The left side 石 means stone, and the right side 介 looks like a person between boundaries. Imagine a stone wedged between two other rocks, hard and unyielding.
For カイ, picture a stone that is so hard it can cut like a knife: knife -> カイ, and the stone is as hard as a blade.