The 'yu' is a beautiful stone ranking just below true jade.
Tips
history
瑀 has no modern standalone use; it appears in classical texts and as a personal-name character. The jade radical 王 (the left-side form of 玉) marks it as a fine stone; 禹 supplies the sound.
register
Archaic and literary. One old dictionary marks the meaning as uncertain, but the jade-stone sense is the attested one. You will only meet 瑀 in old texts, names, and etymology notes.