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A mood ring is a ring that contains a thermochromic element, such as liquid crystal, that changes colors based upon the temperature of the finger of the wearer. Most new rings come with a color chart indicating the supposed mood of the wearer based upon the colors indicated on the ring.
The mood ring was created in 1975 by two New York inventors, Josh Reynolds and Maris Ambats, who bonded liquid crystals with quartz stones set into rings. They initially retailed for $45 for a "silvery setting" and $250 for gold, and first sold at Bonwit Teller, rapidly becoming a fad in the 1970s.