LOL @ diamonds. All you people are getting ripped off no matter where you go.
This, 100%. You're also being lied to which is easy because you know nothing.
Factors in diamond price:
Carat weight: sky's the limit. The larger the diamond, the more the cost per carat.
Color: D-Z, D being crystal clear, Z being jet black. Though generally D is the costliest color, there is a class of diamonds called "fancies", where colors other than D (usually in the I to Q range) are preferred. These diamonds which are generally yellows and oranges have consistent transparent color throughout, and can be as high as 50x the price of colorless diamonds. Most of the synthetic diamonds are fancies. The most expensive diamonds are ruby red.
Clarity: Flawless - I2 (clarity goes below I2, but those are not used in American
Jewelry. ...Indian jewelry, yes)
Cut: more facets = higher price
Symmetry: Each cut has optimum ratios for the various dimensions, most important generally being the depth ratio (width to depth)
Fluorescence: a quality of the diamond's chemical makeup that causes it to glow (fluoresce) under certain lighting. Generally fluorescence degrades the value. So-called blue diamonds are highly fluorescent colorless diamonds.
How jewelers lie to you:
The easiest way is with symmetry. A diamond with a very high depth ratio (taller than it should be) looks smaller from the top down. Diamond cutters usually avoid cutting diamonds in this way, because they are harder to sell to anyone. Conversely, a diamond with a very low depth ratio looks larger from the top down, so people can easily be fooled. "My 1ct diamond looks so much larger than Martha's, and she paid almost twice as much!" So what's the problem? Well, fire. Fire is the term used to convey sparkle. Diamonds have very precise symmetry ratios because they sparkle more the closer they are to ideal cut. Most chain and diamond mart vendors do not sell ideal cut diamonds because they look to attract witless people based on price, and find ideal cuts harder to sell. You'd be lucky if you went to most jewelers and were even shown such a diamond.
Clarity. Many jewelers simply lie about it. This si where GIA certified diamonds come into play, and why you pay more for them. A GIA certified diamond is sent to GIA, where it is dispassionately graded according to all of the above factors. Diamonds sent to GIA post retail often come back certified as something far less than what they were sold as. Jewelers will defend themselves by saying this is subjective (which it is), but GIA is and has been very consistent for many years.
Color is harder to lie about, but it is done.
Fluorescence is often lied about and even touted as desirable! While true that fluorescence can mask some problems with color and clarity in lower-end diamonds, in higher-end diamonds it is NEVER desirable.
I can tell you without any reservation that diamonds bought in most chain jewelers, department stores, or discount warehouses are almost certainly the lower end of the spectrum, and whose stats are highly suspect. They will almost always be flatter than they should be and fluoresce.
If I were buying, I would buy rubies and emeralds. The cost of diamonds is far over-inflated due to DeBeers stranglehold on the world market. If they released all of their diamonds onto the market today, the cost would be less than 1/10th what it is today. They artificially prop up prices by limiting supply. Millions and millions of diamonds are kept in vaults in Switzerland. The great irony of course is the man-made diamonds, which have caused DeBeers to resort to defacing their own natural diamonds with laser inscriptions.