ornamental hairpin
珈 refers to an ornamental hairpin, a decorative accessory worn in the hair.
珈 combines 王 (jade) with 加 (add), suggesting a jade ornament added to the hair. The exact historical development is uncertain, but the component structure points to a decorative jade hairpin.
The left side 王 (jade) and right side 加 (add) together picture adding a jade ornament to the hair: 珈 is an ornamental hairpin.
For カ, imagine a car adorned with a giant ornamental hairpin on its hood: car -> カ, and the jade hairpin makes it fancy.