You can go to a the diamond district and buy a loose stone and have it set in any setting that you want and pay a different price then if you go to Tiffany's and by the very same cut, color, clarity for much more.
My SIL had her eye on a princess cut diamond in a simple platinum setting at Tiffany's that cost, an outrageous amount of money (2.10 ct). I had my Jeweler copy the setting and she got a VS1, G color polish girdle perfect cut for a fraction of the cost. Yes it was very costly but still much cheaper then the ring at Tiffany's. When looking to buy a ring you need to know exactly what you are looking at and a good sales person will explain this in depth so that you understand that if the ring is not cut in proportion meaning that they do a spread cut which makes the top of the ring (the table) look much bigger then it really is the ring will not sparkle. If the girdle is too thick or not polished it can cause the diamond to loose it's luster or brilliance. Then you need to know if you want or even have an European cut or an Modern cut diamond (tables are a different size along with different cutting techniques) Then how many facets which again change all the time as different shapes come about.
Most people do not get a VVs1 diamond they tend to get a Vs1 or even an S1. Most don't get a D, E, or even F, color diamond the most common is G, H, I. The average size in a stone is 1/2 to 1 carat that is why Halo's have become so popular. Price on diamonds vary greatly due to where you purchase them and the setting you want them in. If you need a name then your going to pay for it. Not because the quality is better but because it has a well known name attached to it.
I also know of many that will state that their ring is one thing and in reality it's not that at all because do we ever ask to see the paper work that came with the diamond or I should say should have came with the diamond. I've seen women get a large center stone that was a CZ then add in smaller real stones. A ring is a perception of what you believe to be true.
Buy what looks best to your eye. What sparkles in the sunlight. What you will enjoy no matter where it comes from. By the way Costco has some of the best deals on diamond. They even back this up by saying that the ring will appraise for double the purchase price or you can bring it back no questions asked.