jade look alike stone
瑨 refers to a stone that resembles jade, a jade-like mineral.
瑨 combines 王 (jade) with 晋, which likely contributes the sound. The character denotes a stone similar to jade.
The left side 王 (jade) tells you it's a type of stone, and the right side 晋 looks like a precious object. Imagine a stone that looks so much like jade it could advance (晋) into the jade collection.
For シン, picture a thin (シン) slice of this jade-like stone: thin -> シン, and the stone is so thin it's almost transparent.
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