the top gem of the pendants
珩 refers specifically to the top gem or jade piece in a set of ceremonial pendants, often worn as an ornament. Its meaning is narrow and concrete, centered on this decorative object.
珩 combines 王 (jade) with 行, which likely contributes a phonetic element. The character is associated with a specific jade ornament, but the exact historical development is uncertain.
The left side 王 (jade) and the right side 行 (going) suggest a jade piece that goes at the top of a pendant set. Picture a shining jade gem leading the way as the top ornament.
For コウ, imagine a court official (公, こう) wearing the top gem pendant as a mark of rank: 公 -> コウ, and the gem sits at the top.