The "stone" of the mood ring, which supposedly changes colors as your emotions change, is composed of photographic paper similar to that used in the construction of digital thermometers that change color as temperatures fluctuate.
Iowa Science Teachers Journal
© Copyright 1977 by the Iowa Academy of Science
Iowa Science Teachers Journal: Vol. 14
, Article 9.
Available at: https://scholarworks.uni.edu/istj/vol14/iss3/9