I think most of us figured out when we were children that mood rings do about as good a job at predicting moods as conspiracy groups do at predicting the end of the world. Maybe you also suspected that the ring was temperature rather than temperament-responsive, and performed some experiments to test your hypothesis. At room temperature, the ring was amber. You put it on your finger and it turned blue. You set it in the sun and watched it turned green.
So how do they really work?
A mood ring consists of a thermotropic liquid crystal encased in glass or quartz. Color changes in the liquid crystal respond to changes in temperature. Typically, the colors in the liquid crystals are “calibrated” to the average skin temperature of 37 degrees Celsius so that deviations towards higher and lower temperatures produces different colors.
The liquid crystals in mood rings are not unlike those utilized in the liquid crystal display (LCD) monitors in our laptops. These “crystals” consist of long, rod-like molecules which are loosely ordered by intermolecular interactions into a parallel-packing pattern as shown below. However, the molecules maintain some freedom of motion within the lattice. These ordered structures retain the phase behaviors of both crystals and liquids, earning themselves the name liquid crystal.
The liquid crystal phase encompasses a number of possible subphases, each with varying degrees of orderliness. For thermotropic (temperature-sensitive) liquid crystals, the molecules will coil or tilt as the temperature is fluctuated. These changes in structure produce changes in the liquid crystal’s light absorption properties, and thus we see a continuous evolution in color. For mood rings, this color progression generally proceeds from yellow to violet as the temperature is raised.
Picture 1 source: http://images.sodahead.com/polls/003716463/3621465620_Mood_Ring_xlarge.jpeg
Picture 2 source: http://www-g.eng.cam.ac.uk/CMMPE/images/lcphase.jpg
Information source: http://chemistry.about.com/od/chemistryfaqs/f/moodring.htm