In recent times the popularity of promise rings (commitment ring) is very much on trend. With an engagement ring there can little doubt of the intention of the couple. With a promise ring it is not so clear cut. The reasons for giving and getting promise rings vary, and often only the wearer and giver know what the promise is. What the promise ring symbolizes between couples is highly individual, but generally it represents love and commitment. It could be a promise that after graduation or when it's easier financially they will get engaged.
Promise rings date back to Roman times and during Victorian and Edwardian eras. Gemstones such as opal, amethyst and emerald were very polular.
A good point to keep in mind while choosing a promise ring is, don't buy it with a huge stone in the middle because when the actual engagement is being purchased the recipient might expect the same or a more spectacular ring to be given. A simple classic setting and design will perhaps be a better option. Normally the focus is not on the diamond, but on the overall design.
Metals, too, are often different. While 18K and 14K gold and platinum are popular for engagement and wedding bands, 10K or steling silver are more popular options for promise rings. A Claddagh ring is often a unique and interesting option. As the Claddagh ring is a tangible symbol of love and commitment.
When the engagement ring comes along the promise ring often moves over to the opposite hand. There are no hard and fast rules how the promise ring is worn. Some people choose to wear it on a chain around their neck.