Estee Lauder's Mood compact from cavewoman's collection:

A mood ring is a novelty ring which changes color in response to body temperature, using a thermochromic liquid crystal. A form of biofeedback, they supposedly indicated the temperament of the wearer, indicated by the ring's color. Mood rings were a fad whose popularity peaked in the United States in the 1970s, and they are now seen as an icon of 1970s culture.

The Mood Ring was invented by Joshua Reynolds, who also invented the Thighmaster. Reynolds is heir to the fortune of Richard Joshua Reynolds, tobacco company founder.

The meanings of the various colors:
Black: Tense, nervous, harassed, overworked
Gray: Anxious, nervous, strained
Amber: Nervous, emotions mixed, unsettled, cool
Green: Average reading. Active, not under great stress
Blue-green: Emotionally charged, somewhat relaxed
Blue: Relaxed, at ease, calm
Dark blue: love , romance

photo: cavewoman/info from wikipedia