After Urishama opened the jewel box, it showed regret. It represented regret because now he is old, and weak but also is no longer able to visit the Princess in his kingdom. Once he opened it, he would want to take it back.

Before Urashima opened the box, it represented mystery, and temptation. It represented mystery because he knew that what is in it, will be very important, but also he could not open the box because there would be consequences. His inability to open the box added interest to it. The jewel box showed temptation for the same reasons, he was tempted because he knew it would be something important.

Jewel box

Symbol from Urashima Taro and the Princess of the Sea