coarse variety of jasper or jade
琇 refers to a coarse variety of jasper or jade, a type of stone valued for its appearance.
琇 combines 王 (jewel) with 秀, which likely provides the sound. The exact historical development is uncertain.
The jewel radical 王 next to 秀 (excellent) suggests a stone that is excellent but coarse: a rough jasper or jade.
For シュウ, imagine a shoe made of coarse jade: shoe -> シュウ, and the stone is rough but still a jewel.