type of jade, pearl
珹 centers on a specific type of jade or pearl, referring to a precious stone or gem-like object.
珹 combines 王 (jade) with 成, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a type of jade.
The left side 王 is the jade radical, and the right side 成 looks like a completed piece. Imagine a finished, polished jade pearl.
For セイ, picture a jade pearl being carefully set into a ring: set -> セイ, and the pearl is placed perfectly.