beautiful jade, precious stones
琨 centers on a type of beautiful jade or precious stone, often used ornamentally.
琨 combines 王 (jade) with 昆, which likely contributes the sound. The character is used for a type of beautiful jade or precious stone.
The left side 王 (jade) tells you it's a precious stone, and the right side 昆 looks like a shining sun 日 over a pair of legs 比, as if the jade is so beautiful it glows like the sun.
For コン, imagine a beautiful jade stone shaped like a cone: cone -> コン, and the jade's smooth conical shape makes it even more precious.