The origins of the 'Promise Ring' have  evolved from the wearing of  'Betrothal Rings' in the ancient times, as betrothals were considered a vital part in social life, but quite distinct from engagements by modern definition.
In Biblical terms, a betrothal was an irrevocable and legally binding contract concluded through a public announcement. The earliest recordings of the betrothal rings (George Frederick Kunz) reveal that they were given exclusively by the man during the ceremony and not exchanged as are Promise Rings.  An interesting and later version of such rings first used during 16th century Europe, was the 'gimmal' ring, consisting of three separate hoops.  During the betrothal ceremony, one ring would each be worn by the bride-to-be, the bridegroom-to-be and the 'witness' to the promise as well as the ceremony. During the wedding ceremony, the bride would wear all three parts of the gimmal Promise Ring.
Today, Promise Rings are exchanged for a variety of reasons; Pre-engagement Promise Rings are given prior to engagement but without an official commitment. Purity Promise Rings symbolize a commitment or promise to remain chaste.  In certain European cultures, prior to the liberalization of sexual behaviors, it was customary for a parent to give a purity promise ring to a child symbolizing a promise to remain sexually pure until marriage.  Purity promise rings were also given to symbolize, abstinence from certain bad virtues. Friendship Promise Rings are not related to a romantic affair, but rather a commitment to genuine friendship while Religious Promise Rings indicate a lifetime commitment, worn by a group of believers of a certain faith, united in their decision to follow the teachings or beliefs of that faith.