Gimmel rings, a. A couple would wear an engagement ring each during their engagement. A couple would wear a diamond ring each during their engagement. Go for gems that are rarer than diamonds.
Most importantly though, it says that you are committed to everlasting beauty, a beauty that goes deeper than looks, a beauty that can be as eternal as a diamond. The look of the engagement ring is influenced mostly from the shape of the diamond. Use different gems with different colors too. The sharpest point is considered the most ideal, though the actual cut may differ depending on the taste of the individual cutting the stone so feel free to choose what looks far better to your eye.