The first well documented use of a diamond ring to signify engagement was by the Archduke Maximilian of Austria in 1477 to Mary of Burgundy. However engagement rings began in Ancient Egypt as a band, and the circle symbolize a never ending cycle.
The ring was placed on the forth finger on the left hand, because Ancient Egyptians belived that it contained a vein that lead to the heart.
Betrothal rings were used during Roman times and they included a small key, romantics belive that the carved key was used as a symbol to protect and cherish the husband’s heart.
Greeks also used betrothal rings but they were not required to be given before marriage, unlike traditional engagement rings today.
The use of a diamond as the most popular choice for an engagement ring is because a diamond is said to be forever due to its durability and therefore its association with love and marriage that are meant to last forever.