The first and most important quality of a diamond is the carat weight. A carat is defined to be 0.2 grams. As weight increases, so does the carat size.
The next most important quality to consider is the clarity. The creation of a natural diamond takes millions of years and high pressure, and during this formation it is typical that particles get stuck in the diamond, these are called inclusions. In order to better differentiate between the number of inclusions, they're graded on a scale:
Diamonds can come in many different colors, this happens during the formation of the diamond when other compounds interact with the carbon molecules being formed. They too are also graded on a scale ranging from colorless to yellow:
The cut of a diamond is extremely important, it determines how well your diamond will reflect light, and therefore how sparkly it is. There are three factors to consider when looking at the symmetry of a cut, the crown, the pavilion, and the depth. If all three of these are perfectly symmetrical, you get an ideal cut.