A mood ring, popular in the mid-1970s. Photograph by Dan Newell. Reproduced by permission of Leitha Etheridge-Sims.

Mood rings were just that—rings that indicated the mood of the person wearing it. At least that was the promise. Mood rings were one of the many fads that briefly captured people's attentions during the mid-1970s. But more than just a fad, they were a perfect symbol for the decade, a time when people looked inward to their own lives after a decade of social turmoil in the 1960s.

Mood rings were the invention of Joshua Reynolds. In the early 1970s, Reynolds got involved in a number of projects designed to help people discover their true feelings and understand their emotions better. Among these projects were biofeedback, which used a machine to monitor brain waves, and a meditation center called Q-Tran, which used the biofeedback machines...