It's the color. The further down the alphabet, the less white the color. I have a 1.02 carat I actually bought myself. It has a very thick girdle but even with a 40x you can not see inclusions, they are hidden by the cut. This diamond should not be worth all that much, but it is very very white and has fluorescence. I have another that has a beautiful cut, extremely clean, but color is a little lower down, about an H or J or so. While the other one's cut is very much appreciated, it is this one that attracts all the attention because it is so white, so fiery, and I have had some of the best jewelers in town take a peek at it for me just for an opinion. It always grabs them, and they stare at it, and regardless of the thick girdle, etc. there is something magical about it and they just gaze at it. When I go out, it just flashes like crazy. What I am saying is after 4 big engagement rings and after having been married to a gemologist (and divorced), I will say that color is what makes the ring spectacular instead of just technically amazing. And it was between this one and a Hearts on Fire which had a depth like water, but this one just had that amazing whiteness, brilliance, and while half the worth of the Hearts on Fire, it was just the most beautiful diamond ever in my collection. And I like the fluorescence. It is really about what appeals to you, but color is number 1 for me, then cut. If you are not in love with your diamond, look at exchanging it! Costco has some amazing diamonds, but I got mine at an estate auction, and was super lucky!