Maria Doulton: Not to be forgotten is another vital 'C', and that is of course carat. Carat refers to the weight of the stone, and one carat is one fifth of a gram. Now, you don't need me to tell you that a large diamond is more valuable than a small diamond, but it is interesting to know that large diamonds are of course rarer, so this means that a 5.00 carat diamond is many times more valuable than a 1.00 carat diamond; it isn't simply five times more valuable.
Not all diamond cuts look the same size even if they are the same carat weight, and a brilliant cut is the one that punches above its weight because of its diameter; it looks bigger than a similar-carat emerald cut. If you're going for the big look on the finger, the brilliant cut will do it every time.
Now, for a single-stone ring, for an engagement ring, I think don't go below 1.00 carat because you really want to have that wow factor. Carat and size is not everything, so you always have to balance that with the other three Cs, and come to the ideal choice that works just right for you. Bear in mind that a well-cut diamond will always look better than a lesser-quality cut, even if it's the same carat weight.
Now, Graff, who are absolute experts at diamond cutting and polishing, will make the most of every single diamond that they sell, so it really is worth going for the best quality of cut if you want to make the most of the size of your diamond.