So we know how they work, but do mood rings really work? Do they really tell your mood?
Short answer? Nope. They are not able to tell you mood, BUT....
The color change of a mood stone can reflect temperature changes caused by the body's physical reaction to emotions. When you're anxious, blood is directed toward the body's core, reducing the temperature at extremities like the fingers. When you are calm, more blood is flowing through the fingers, making them warmer. When you're excited or have been exercising, increased circulation warms your fingers.
Green or blue, which signifies calm or happy on the mood ring color scale, is calibrated to the surface temperature of a typical person, approximately 82 degrees Fahrenheit (98.6 internal temperature). If your surface temperature varies far enough from the norm, then the liquid crystals in the stone alter enough to cause a change in the color reflected. And if you take a mood ring off, it will normally change to black unless the ambient temperature is very high.
Modern mood rings don't necessarily turn black. The bottom of newer stones are black so that when the ring loses its ability to change color, it's still attractive.
So mood stones can show which emotion you are experiencing with some accuracy, but mood? Not quite. Think about how quickly you change your mood throughout the day. I don't know about you, but I am pretty moody. I'm not sure any ring could keep up with me!