The mood ring was created in the 1970s by Josh Reynolds and Maris Ambats and was used to communicate the current emotional state of the wearer based on the temperature of their skin. The mood ring is now worn as something of a toy, but in the seventies was taken as a serious piece of jewelry.

unique color properties, cholesteric liquid crystals can be employed to indicate temperature field patterns and for color picture screens

The technology has practical functions in medicine. It can be used to measure temperature fluctuations on specific body parts in order to locate blood clots, find cancer cells, localize placenta, and test pharmalogical drugs (Ericksen 84). This fits in to how the mood ring can vaguely determine mood: "The basic idea is that areas of the body in which circulation is poor will have lower temperatures than areas with good circulation" (Ericksen 84). When a person is excited or stimulated in some way, a common reaction is blushing, which is when capillaries move closer to the surface of the skin and heat it up. In this way, the mood ring is accurate in that it senses a rise in heat which is related to happiness and arousal.

The process has limitations, however. It is delicate; like most temperature-measuring devices, liquid crystals are most accurate when the context of the measurement is not changing. "[L]iquid crystals can be used to measure the absolute temperature of a process provided proper calibration can be performed. [...] Perhaps the most obvious is to keep the system temperature constant" "to make the color change clearly visible, the surface to which the liquid crystal film is applied must be black or first dyed black.This is why mood rings are black when not in use.