For other uses, see Mood ring (disambiguation).A mood ring is a finger ring that contains a thermochromic element, or mood stone that changes colors based upon the temperature of the finger of the wearer. Finger temperature, as long as the ambient temperature is relatively constant is significantly determined by peripheral blood flow, which is modulated by the autonomic nervous system. A mood ring contains liquid crystals which change color depending on the temperature.

The original mood stone was a quartz crystal oval cabochon treated with heat sensitive (thermochromic) liquid crystal material. The mood stone ring was made of sterling silver or vermeil (gold-gilded sterling), whereas the majority of mood rings were based on base metals, such as copper. Changes in peripheral blood flow (thus finger temperature) cause the liquid crystal to reflect different wavelengths of light which changes the color of the stone. The liquid crystal used in the original mood stone ring was engineered to display a range of 7 distinct colors spanning over a 20 °F (11 °C) temperature range. A black stone reflects cold hands. As finger temperature warms, with more blood flow to the extremities, the mood stone's color progresses from brown to yellow to light green to dark green to light blue to dark blue.