Mood Rings are one of the most unique pieces of jewellery

How It works: 

The thermal chromatic liquid inside the ring reacts when a surface is Cold or Hot... This makes the ring change colour and it shows off how you are feeling by the temperature of your body!   The inside of the ring conducts heat from your finger to the liquid crystals in the "stone." The colour  green, which signifies "average" on the mood ring colour scale, is calibrated to the  surface temperature of a typical person, approximately 82 degrees Fahrenheit (28 degrees Celsius). 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. This physical reaction to your emotional state causes the capillaries to move deeper into your skin, causing the surface temperature to drop.