The Four C's: Carat, Clarity, Color and Cut.
Color is a fun area, as diamonds can actually be many colors, even black. These are deemed "fancy," such as the pink diamond shown above. All other diamonds can have different amounts of inclusions that affect their color, "D" being the most colorless.
I think if I were proposing the trickiest part would be the cut. Mainly because this is only based on your fiancé-to-be's preference. Ask her best friend, her parents, or watch her closely for her preferences...and don't feel that you have to propose with the final ring, you can have an inexpensive "work in progress ring" for the night of.
The ring pictured in row two is my "dream ring" created from Abazia's Ring Builder, which is pretty awesome. It allows you to shop from over 100,000 diamonds and then preview them in your band. I had a little too much fun I'm afraid as my ideal ring cost $63,000.
Top: ring in box photo courtesy of Martha Stewart, two sparklers photo courtesy of Martha Stewart, engagement rings photo courtesy of The Knot, stacked diamond rings photo courtesy of Martha Stewart. Second Row: dream solitaire ring available through Abazias, cluster ring photo courtesy of Martha Stewart, wedding bands on apple photo, photo courtesy of Ann Hamilton, loose diamond, photo courtesy of Abazia's diamond blog, pink diamond, photo courtesy of the Natural Color Diamond Association.