There are four major factors to take in to consideration when determining a diamond's value and pricing it. This is known as "The 4C's". Once you understand them, it will be easier for you to choose the diamond you want.

Carat
Refers to a diamond's weight.
The weight of a diamond is expressed in carats, when, 1 carat equals 0.20 grams.
One carat is divided into 100 points, meaning a quarter of a carat is 25 points.
Most people compare carat weight to size. The larger a diamond is, the more it weighs.
When mining the diamonds it is far rarer to find a big diamond than small ones, therefore, the higher the carat weight is, the higher the price is.


Cut
Refers to a diamond's proportions, symmetry and polish.
The cut of a diamond has a large effect on its appearance, the higher the quality of a diamond's cut is the more it will sparkle.
Based on mathematical formulas, an ideal cut diamond will reflect light from one facet to another and will disperse trough it's top, resulting in a magnificent display of brilliance and fire.


Color
Refers to the degree of which a diamond is colorless.
Diamond color has a significant impact on its value.
The color is ranged in alphabetical order from D to Z, when D is colorless and the most expensive, going down to Z which is light yellow.
The differences in color are very slim, for example:
The difference between D, E and F color are hard to distinguish even trough the eye of a professional.


Clarity
Refers to how much a diamond is clean and clear.
Diamonds that are absolutely clean are the rarest and therefore the most expensive ones.
Most diamonds contains natural flaws such as traces of minerals trapped inside the diamond.
They may look like tiny crystals, clouds or feathers.
These flaws detract from the diamond's value, purity and beauty.