This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
瑆 combines 王 (jade) with 星 (star), likely indicating a type of jade or a star-like quality of jade, but the exact historical development is uncertain.
The left side 王 suggests jade, and the right side 星 suggests a star. Imagine a piece of jade that shines like a star, but its exact meaning remains unclear.
For セイ, picture a star (星, せい) made of jade: 星 -> セイ, and the jade star glows with a pure light.