April Birthstone
While April's birthstone is either the diamond or the sapphire, they do say that diamonds are a girl's best friend.
Diamonds represent faithfulness, purity and innocence so it is understandably a stone of choice for an engagement ring.
Do keep in mind that a good alternative to the diamond is a cubic zirconia and I can speak from personal experience. As I often lose at least one earring from a set I now buy cubic zirconia and it looks just as sparkly and impressive.