two hemispherical objects
珓 refers to two hemispherical objects used together, likely as a pair of divination blocks or similar ritual items. The meaning is narrow and specific to this paired-object concept.
珓 combines 王 (jade) with 交, which likely contributes a phonetic element. The character probably originally referred to jade or stone objects used in pairs, but the exact historical development is uncertain.
The left side 王 suggests jade or precious stone, and the right side 交 suggests crossing or exchanging. Imagine two hemispherical jade pieces that fit together when crossed or brought into contact.
For コウ, picture a pair of hemispherical objects clinking together with a hollow 'koh' sound: koh -> コウ, like two stone cups meeting.