I think you need to figure out what you are wanting more. A round shape is more expensive than a square shape. Also there are many types of square shaped diamonds: emerald, princess, and radiant shapes. Is this an engagement ring diamond? If so I would recommend SI1 for stones above about 0.5ct. Emerald shapes can show inclusions easier, you may even want VS2 if you decide on this shape (though I know little about shapes that aren't rounds). Also you can drop down to a G-H color and most people will not be able to tell the difference.
What kind of cut (not shape) do you want? The cut determines how the diamond shines and sparkles. Stones with extremely precise symmetry display a Hearts and Arrows (H&A) pattern (note that a H&A diamond can still have bad proportions). AGS is a labratory that certifies diamonds; an AGS 0 cut means it has excellent polish/symmetry/proportions. The better cut stones demand a higher price.
Also, an excellently cut stone will not show its color as much. I decided to go with a J color stone with an AGS 0, H&A cut.
All that said, to get ideas of the price ranges we are talking, check out the PriceScope.
Ahem,.... Nate,.... Dear,.... Sweetie,..... I have a 100% eye clean .766 carat eye clean SI1.
Your question is very complicated.
Rounds cost more than other cuts generally speaking. There are exceptions to every rule. I'm sure there is probably some exotic pear or emerald cut out there worth a fortune. Rounds tend to be the most sparkly even in a so so cut. Rounds are also well loved and wanted by lots of people. Soooo there is a market driving force on the pricing as well. While lovely diamonds, squares are not as popular as rounds. Sooooo, squares are priced somewhat lower than rounds. Partly market driven, but also performance driven as well. Squares are hard to find with a lot of brilliance, fire, and pizzazz.
Cut is a huge factor of price in any diamond. You can have 2 F, VS1 same shape, same carat weight diamonds. One will be $1000 while the other is $1500. The $1000 one could be deep, leaks light due to thick girdle, has a large table, on and on. The $1500 one was cut to Ideal specs maximizing size and visual performance. That's just an example, but I hope you get the idea.
Color impacts price. D/E/F are colorless and bring the higher prices. D especially. G/H/I are near colorless and can offer the best bargains for folks wanting colorless but willing to compromise some to get the size they want. With an Ideal cut round, you can easily drop to J and the diamond will face up well even in white gold or platinum. Squares hold and show more body color. You'd need to go higher in color with a square than you might consider in a round.
Clarity impacts price. Flawless are extremely rare and priced accordingly. The larger the diamond the higher the price for Flawless. VVS are more readily available, but still pricey. VS even more available but getting more *economical*. SI's are tricky, but can get you the biggest bang for your buck in clarity. SI1's tend to be eye clean, but not always, especially in larger diamonds. SI2's are tricky, but can be eye clean. I1's offer great bargains. SI's and I's all depend on the type, size, and location of the inclusions.
To answer your question, I did a search of Round Brilliants. On the Price Scope homepage, left hand side search engine. I clicked H&A, entered D and F for color, .95 to 1.05 for carat weight, and SI2 only for clarity. Prices ranged from around $4300 to nearly $5800. The biggest difference in the prices of similar color and carat weight diamonds was CUT. And CUT has a HUGE impact on how the diamond looks and performs visually. Does it just sit there, or does it flash the colors of the rainbow.
Guess I should've been more clear... I recommended SI1 or above because I've seen plenty of SI2 stones that weren't eye clean. Granted I've seen plenty that were, but in my own diamond quest I decided that since I'm looking for a really good price it was quite likely that the best priced SI2s aren't eye clean. I figured its a safer bet just to go for the SI1. SI2s can definitely be eye clean, I didn't mean to offend, PQ!
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