Outside America, wearing a ring on the thumb is surprisingly common. If you wear mens rings on this finger, it’s a sign that you’re a man of great wealth and influence. Thus, thumb rings tend to have bold—but not loud—designs and thick bands to fit an equally thick finger.
The thumb is the only finger without a god/planet associated with it. However, in ancient times, it was used as a gauge of someone’s character. Straight thumbs indicated a strong personality, while crooked thumbs indicated an equally crooked person.