name of a kind of jade
珣 is a rare kanji that names a specific kind of jade. Its meaning is limited to that ornamental stone reference.
珣 combines 王 (jade) with 旬, which likely contributes the sound. The character was created to name a specific type of jade.
The left side 王 (jade) tells you this is a precious stone. The right side 旬 (ten-day period) can remind you that this rare jade is only found every ten days, making it special.
For シュン, imagine a shiny jade ornament that gleams like the sun: shiny jade -> シュン.