I guess I'm in a 70's mood!
A mood ring is a ring which contains a thermochromic element, such as liquid crystal. The ring changes color in response to the body temperature of its wearer. The color is said, by some proponents, to indicate the emotional state of the wearer.
Changes in temperature cause the crystal to reflect different wavelengths of light which changes the color of the stone. First sold at Bonwit Teller in New York City, the Mood Ring rapidly becoming a fad in the 1970s.