What material is a "mood" ring made of that changes colors with (body) heat?

Answer 1

Most mood rings incorporate temperature-sensitive liquid crystals which change color as your body temperature changes during the day. These same liquid crystals are used in the stick-on thermometers for aquariums, or the temperature strips that are sold to measure fevers in young children, etc.

For more info, look here:

http://home.howstuffworks.com/question443.htm
http://ask.yahoo.com/20040902.html
http://en.wikipedia.org/wiki/Mood_ring