Mood rings, which gained popularity in the 1970s, cannot accurately identify human moods, but they do change color as the wearer's body temperature fluctuates. The rings contain liquid crystals that twist and change color in response to temperature changes.
A mood ring cannot detect changes in an individual person's mood at any given time. The rings are designed to show a blue or green color at the normal human body temperature, changing to various shades of brown, yellow, green and violet depending on the temperature of the wearer's finger.
Sometimes if an individual is in a good mood, her overall body temperature rises. This can cause the ring to turn a bright blue. Alternatively, if an individual is psychologically or physiologically stressed, the body may redirect blood away from the extremities and toward the core. This can decrease the overall temperature of the finger and cause the crystals to turn yellow or orange. In this sense, the mood ring does have the potential to indicate shifts in a person's mental state, but it is far from a reliable science.
A mood ring can also show colors that demonstrate a blended effect, such as green fading into yellow as the finger's temperature changes. If an individual wears the ring outdoors in the cold, it may display a black color and become non-responsive. This also occurs if the ring is broken.