A mood ring is a ring which contains a thermo chromic 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. (Wikipedia)
These fun rings were a big craze in the 1970's and can still be found in gift shops today.
The examples below are from the 1970's and are new old stock from a closed up jewelry store - they have never been worn. The glass thermo chromic cabochons are set in gold tone metal with adjustable bands. Original acrylic case marked Living Gem with a sticker on the bottom that says Made In Hong Kong. Each comes with instructions for care and a guide to "mood color"