It refers to the shape, proportions and characteristics of a diamond and all these factors go into determining its brilliance.
Most diamonds have some inclusions or internal flaws. The number and size of these inclusions determine the clarity of the diamond. A diamond can have clarity ranging from internally flawless (no inclusions) to inclusions that are easy to locate with the naked eye. As the clarity of the diamond decreases, so does the price.
Diamonds have colors ranging from colorless to near colorless and are usually described as shades of yellow. Diamonds come in many different colors, such as pink and blue.
A diamond is weighed in carats. Since large diamonds are much more rare, the price of a diamond is directly related to its carat size.