It wasn't until the early 1900's that the use of diamond rings began surfacing with regularity. The circle being a symbol of eternity wasn't enough as diamond cartels began advertising that “a diamond is forever.”
They had many people believing that if someone didn't provide a diamond ring when proposing that it was a shameful act. Some women believed that the size of the diamond was an indication of their suiters love and commitment. You've got to hand it to the diamond cartel, they did a number on us. :)