This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
硈 combines 石 (stone) with 吉, but the exact historical development and original meaning are uncertain.
The left side is stone 石, and the right side is 吉 (good luck). Imagine a lucky stone that is hard and solid, but its true meaning is unclear.
For カツ, picture a lucky stone that cuts through other rocks: cuts -> カツ, and the stone is hard.