polish, brilliant white luster of a gem, artful smile
瑳 centers on a bright, polished surface that gleams with a clean white luster, like a finely polished gem. The sense of an artful smile extends this idea to a radiant, polished expression.
瑳 combines 王 (jade/gem) with 差, which likely contributes a phonetic element. The character is associated with the bright luster of polished jade.
The left side 王 (jade) is being polished by the right side 差, which looks like a tool with a rough surface. Imagine rubbing a gemstone with this tool until it shines with a brilliant white luster, so bright it makes you smile artfully.
For サ, picture polishing a gem so perfectly that it reflects the sun (サン) like a mirror: sun -> サン, and the first syllable サ sticks as the gem's brilliant luster.