There are many different promise ring styles:
White Gold Promise RingsDiamond Promise RingsYellow Gold Promise RingsSolid 14K designsMany designs contain the heart symbol and it is symbolic in that the lover dedicates their heart to being with each other. Sterling silver promise rings are also given when the purchaser is unable to afford the precious syles and genuine diamonds.
Other designs don't necessarily contain hearts. They are usually simplistic in design and may have diamonds. Many use 3 stone diamond rings (usually smaller in stone size than the engagement three stone) or another classic design.
Promise rings date back to the 16th century. They were also known as friendship rings and illustrations show these rings (as early as 1576AD). Back in the day, when a man could not afford to get married as the social structure of marriage was forbidding, they would buy a promise ring as a placeholder until such time they would be able to effectuate a marriage.
During the 20th century, in some instances women would offer men's promise rings to their man because, at the time, he was unable to commission to be wed. It was generally a surprise offering and was something the man could accept or deny.
Tradition generally holds that these rings were given by younger couples. This was usually the case because it was normal for the young couple not to have a wealth position that would allow for marriage and children. Nowadays anyone can give a promise ring, regardless of wealth or social status.
References:
Ades, Michael "Beautiful Promise Rings" A True Love Commitment Spring 2002
