Started from the Romans and Egyptians engagement rings are band that seals love of human erotically attached. Romans taught us how to be generous enough towards the person we love, the Egyptians told us how to placed everything in symbols. The Romans used engagement rings to show ownership because of bethrothal while the Egyptians let us knew that left finger of the left hand has a vein that travels directly to the heart. The Romans introduced engagement rings as plain iron hoop same as with Egyptians. In a larger sense these two great ancient civilization taught us what real love is.
In the ancient times engagement rings are simple and looked like a clasped hands that without the other it will look incomplete. The same as people in love. Without her/him it seems you get lost. Why? Because of the belief that when two people are in love their hearts and soul become one. Once they are married there will be a single light travelling from Earth to the hights of the mountain.
As of the new generation engagement rings still plays it role. How we appreciate it was due to its rich history that even millions of generation will come, the value would still be the same.