sound of stones knocking together
硜 centers on the hard, sharp sound produced when stones strike each other.
硜 combines 石 (stone) with 巠, which likely contributes the sound and reinforces the idea of hardness or a stone-like quality. The exact historical development of 巠 is uncertain.
The left side 石 means stone. The right side 巠 looks like a series of hard, angular lines. Imagine two stones with this jagged shape knocking together, producing a sharp clacking sound.
For コウ, picture two stones knocking together with a loud 'ko' sound, like the start of 'cobalt' but harder: ko -> コウ. The echo of the knock reinforces the reading.