There are LOTS of cushion cuts out there, I think you have to decide is that what you really want. Cushion cuts can sometimes look smaller than say a round, because of the nature of the cut itself.
However, you have to decide if thats something you really want.
Rounds tend to be about 20% more expensive than other cuts (exception are french cut stones) because of their popularity. Its certainly not because they're difficult to cut!
Also, with cushions, while you do get sparkle, the shape itself lends itself to flashes more than sparkle. Your best bet is to look at the table size. Table size will tell you if you get more or less flashes. A larger table gets more flashes and less sparkle, a smaller table will give more sparkle and less flashes.
Make sure the diamond is not too deep. In the world of diamonds, it makes very little sense to pay for something you can't even see. Diamond depths generally are much shallower than colored gemstone because for CG's, you cut for color.
The one thing you want to make sure of, if you're into icy white stones, is that it faces up that color. This is something many won't tell you, because they beleive that you MUST have a F or better.
No, not unless you're very sensitive to color and/or thats the ONLY way you can insure you get an icy white stone.
For sake of affordability, get a stone that has faint to medium blue Fluorescence which really kicks the whiteness in the face up position. So what may be graded as say I-J color can face up as a E-F. To know the true color of a diamond, they are graded face down.
Be careful of too much blue Fluorescence as it can make the stone look oily in bright light.
Another thing to not worry about are inclusions that you cannot see. Inclusions are a stone's birthmark. Personally l like knowing mine has them because that tells me that the stone is definitely mine. If you have your heart set on flawless, you're going to pay the flawless premium. The only thing you should be wary of are black carbon spots, and feathers or naturals that could possibly compromise the structure of your stone. All other inclusions, as long as you can't see them, don't worry about them.
Many stones will have clear naturals in them. As long as they're not eye visible, its nothing to worry about.
Your stone should be GIA graded. For an e-ring, get nothing less. GIA is the most stringent gradings for a stone, and if you're going to get your ring insured, you MUST have the stone graded. EGL , IGI, tends to be the less stringent grading. They will sometimes grade a stone much higher than it should be. You can buy an ungraded stone, however, you should make sure that the place where you buy the stone will guarantee the color and, take it back if you send it to GIA and it doesn't come back as the grade they say it is.
I would check out good old gold's website because they have the actual stone, the ASET and other information of the stone. They also have some of the best cushion cut diamonds I've ever seen
www.goodoldgold.com
-A