You would think is a simply answered question; maybe each 1ct. diamond has the same price tag, no matter which stone you choose? That would be very wishful thinking.
Imagine going to the supermarket to buy apples for your next door neighbor. You get to the fruits and veggies aisle and lo and behold there are 6 different types of apples in front of you.
Green ones, red ones, yellow ones, big ones, small ones, and each with a different price tag attached. This is similar to picking and pricing out a 1ct. diamond.
Prices vary significantly depending on a few basic characteristics.
This first important factor to keep in mind, especially when looking at 1 carat stones, is the make or cut grade. The cut grade simply means how well it was cut and polished and it is graded on a scale from poor to excellent. Poorly cut stones may have funky facets and less symmetry and sparkle while an excellently cut diamond will have precise facets and be perfectly shaped. Round diamonds are the only shape to get an official cut grade from the GIA, but we can look at the polish, symmetry and proportions to determine how well cut a fancy shape stone is going to be.
The shape of the diamond is what puts the stone in a particular range of prices. Diamonds, like stocks or other commodities, are priced based on their desirability. The prices of diamonds goes up and down (like the markets) but a few key things remain the same. For example a round 1ct. diamond will always cost more than say a princess cut diamond (or any fancy cut diamond for that matter) with the same color and clarity.
This brings us nicely into the next two characteristics, you guessed it, color and clarity. The color and clarity of a diamond are what make it beautiful for our eye to look at. They have an effect on the price of all diamond sizes, but especially around that 1ct. range.
Below you can take a look at two diamonds, both 1ct on the scale. The one on the left is a 1ct. K/SI2 while the one on the right is a G/VS1.
You can see that the look and price of these two stones is very different although they weigh the same on a scale.
Another thing to take into consideration when buying a one carat diamond is whether you are buying it set in a ring or loose. Many times the price of a stone will go up once it is mounted in a piece of jewelry because it becomes more unique when combined with precious metal and possibly other diamonds. When buying a loose diamond, the price is based solely on the stone itself and its visual desirability and grade.
Although diamond fluorescence can visually be hard to pick up on at times, it is another characteristic that may affect the price of your 1ct. diamond. Too strong of fluorescence in a DEFG color grade and you'll see a price dip, while stronger fluorescence in an H or lower and you may see the price rise.
Overall, the less color a diamond has and the fewer inclusions the higher the price per carat will be. Combinations of a lower color and higher clarity or higher color with fluorescence for example, are good ways to find a beautiful 1ct. stone that catches your eye and is within your budget.