mother-of-pearl
珧 centers on mother-of-pearl, the iridescent inner layer of certain shells, often used ornamentally.
珧 combines 王 (jade/jewel) with 兆, which likely contributes the sound. The character refers to mother-of-pearl, a jewel-like shell material.
The left side 王 (jewel) and right side 兆 (portent/omen) together suggest a precious, shimmering material: mother-of-pearl, like a jewel from the sea.
For ヨウ, imagine a glowing, iridescent mother-of-pearl shell that reminds you of a yo-yo: yo-yo -> ヨウ, and the shell's luster is as captivating as a spinning yo-yo.