First you have to consider the following: Color, Clarity, Cut, and Carat.
The closer to colorless a diamond is, the better quality. Colors D, E and F are the highest grade. G, H, I and J are next up. Anything from K to Z is tinted.
Clarity: This is determined by the size and number of inclusions in the diamond. This is term means anything that will interfere with a direct path of light.
Cut: This is what changes the sparkle of the diamond.
Carat: This is the weight of the diamond in grams.
With all this being said, you want a ring that represents your personality. This is something that you will wear the rest of your life. Make sure you love it, but also keep in mind that your fiance is the one who put a lot of heart into picking it out.