They are
Cut, Carat weight, Color and
Clarity. Together, they determine the value of a given piece of diamond jewellery.
Cut: Each diamond is cut using an exact mathematical formula to unleash its brilliance. The most common cut is round brilliant, which has 58 flat, polished facets that reflect the maximum amount of light. The cut should not be confused with the shape of the diamond such as marquise, oval, princess (square), etc.
Carat: The weight of diamonds is measured in carats. One carat of diamond is equal to 200 milligrams and there are 142 carats to an ounce. One carat is made up of one hundred points. When one buys a jewellery piece that is made up of many small pieces of diamonds such as a necklace, bracelet or earring, the combined weight of all pieces in the article is indicated. However, because larger diamonds are rarer than small ones, they are more expensive. Thus, a single solitaire diamonds of 1 carat can cost 5-20 times more than a diamond jewellery containing many smaller pieces weighing 1 carat.
C0lor:Colorless D-E-F Yellowish U-V-W
The diamonds can have different colors. The most valuable diamonds are colorless and they are extremely rare. As a result they are quite expensive. The most common color present to varying extents in most diamonds is yellow. The degree of coloration of a diamond is measured on a scale established by the Gemological Institute of America, which ranges from D (colorless) to Z (most yellowish). Color can be best visualized by looking at a loose diamond on a pure white surface and by noting any contrast. The same weight of diamond with D, E or F color will be considerably more expensive than a diamond of U, V, W or X color.
Clarity: The clarity indicates whether a diamond is internally clear or contains internal imperfections and external irregularities that may affect the passage of light. Imperfections such as spots, lines, or bubbles are known as inclusions. A diamond with fewer inclusions is more valuable. Based on a system developed by the Gemological Institute of America, the clarity of a diamond is graded on a scale ranging from flawless (FL or IF) to imperfect (I). The system is based on the visibility of inclusions at a magnification of 10x.
In addition to these four traditional criteria for determining the value a diamond, there is another important criterion "Cut Grade" that has recently been introduced to judge the value of a given diamond piece. The cut grade provides a measure of the brilliance and sparkle of the diamond and in terms of the overall effect of the diamond, it is one the most important indicator.
For a diamond jewellery containing multiple pieces, the above four factors will likely differ from piece to piece. However, in order to assess the quality of the diamonds and overall value of the jewellery, it is necessary to pay attention to the color of the diamond (i.e. whether it is colorless and brilliant or has yellowish dull coloration) and any visible imperfection such as spots, lines, or bubbles that can be easily observed when seen using a magnifying lens.

Sample Diamond Quality Document