Where does “勾玉” come from?

勾玉 (Japanese) comes from Japanese 玉, from Middle Chinese 玉.

Ancestry of “勾玉”, step by step

Step	Language	Word	Meaning
1	Japanese	玉	"any spherical or similarly-rounded object"; a...
2	Middle Chinese	玉	—

勾玉 — Etymology, Origin & Word History | Etymology Explorer