flaw in gem, flaw in character
玷 centers on a blemish or imperfection, whether a physical flaw in a gem or a moral flaw in a person's character.
玷 combines 王 (jade/gem) with 占, which likely contributes the sound. The character originally referred to a flaw or blemish in jade, later extended to moral flaws.
The left side 王 (jade) represents a precious gem, and the right side 占 looks like a crack or mark on its surface. Imagine a beautiful jade stone with a visible flaw that diminishes its value, just as a character flaw diminishes a person.
For テン, picture a gem with a tiny flaw that makes it worth only ten yen: ten -> テン. The flaw reduces its value to just ten.