karat
珖 is a rare kanji whose core meaning centers on a unit of purity for precious materials, specifically karat. Its standalone meaning is narrow and technical.
珖 combines 王 (jewel, a common component in characters related to precious stones or jade) with 光 (light, brightness), likely suggesting a shining gem or a measure of purity. The exact historical development is uncertain.
The left side 王 suggests a jewel or precious stone, and the right side 光 means light. Imagine a bright light shining through a gem to measure its clarity in karats.
For コ, picture a jeweler examining a gem and saying 'co' as in 'co-rrect' when the karat count is right: co -> コ.