Mood rings first became a fad in the seventies. The rings consisted of circular "stones" set on metal bands. The stones were supposed to change color depending on the mood of the wearer. The stones measured the heat coming off the body of the wearer. In theory, this had something to do with their mood, since stress or fear tends to send the blood away from the extremities and cool them down. In fact, the color of the stone probably had more to do with body chemistry and whether or not its wearer had just been holding a cup of hot soup.