Which object is extremely useless, but you just can‘t get rid of it because it reminds you of something?

During the summer of 2021 I went to a club with my best friend. Things were going wrong that day and I wasn’t in the mood. That club sucked as well, so it didn’t help at all. My friend went to the bathroom and came back with two stones, saying “I didn’t know how to cheer you up so I stole these matching stones from the bathroom”. I still keep it and even brought it with me to the Netherlands. As I know, she still has hers too.

Alyona, 23