The 5C's of a certified diamond.

Cut
The cut of a diamond determines how a diamond shines, the cut does not refer to the shape of the diamond. The master diamond cutter has created the facets on the diamond to allow light to internally reflect from one mirror-like facet to another and disperse and reflect it through the top of the stone in a display of brilliance and fire. So when we talk about diamond cut we are referring to the reflective qualities of the diamond which determines its ability to handle light, which leads to brilliance.
As shown in the images below, when a diamond is well-cut, light enters through the table (top of the stone) and travels to the pavilion where it reflects from one side to the other. If the diamond is cut poorly like in the diagrams below, where one is cut too deep and one too shallow, the light will escape from below. But when the cut is ideal, such as in the example on the right, the light reflects back out of the diamond through the table and to the observer's eye. This light is the brilliance we mentioned, and it's this flashing, fiery effect that makes diamonds so mesmerizing.
Colour
The colour grading for white diamonds starts from completely colourless, the most expensive to a faint yellow colour. The colour in a diamond is produced from chemical impurities contained in it's composition. The whiter or colourless the diamond, the more brilliant it will be because it will allow more light to pass and be reflected back. Also the whiter the diamond, the more colours it will reflect back to achieve that desirable rainbow brilliance only a well-cut, colourless diamond can create.
Diamonds graded D - F are the rarest and most expensive, whereas the ones graded G - J are seemingly colourless to the untrained eye. The diamonds with grades K - Z are slightly to more yellowish, however the choice always depends on the buyer's preference.
The chart below shows the colour grading:
Diamonds graded D - F are the rarest and most expensive, whereas the ones graded G - J are seemingly colourless to the untrained eye. The diamonds with grades K - Z are slightly to more yellowish, however the choice always depends on the buyer's preference.
The chart below shows the colour grading:
Clarity
A diamond's clarity is a measurement of the amount of blemishes (external flaws) or inclusions (internal flaws) a diamond contains.
The grading starts at F which is a completely flawless (internally & externally) and extremely rare diamond.
For grades F through SI, a diamond's clarity grade has an impact on the diamond's value, not on the unmagnified diamond's appearance.
Certificates include what is known as a "plot" of a diamond's inclusions -- think of it as a "diamond fingerprint." Since no two diamonds are exactly the same, comparing the uniqueness of your diamond's clarity characteristics with the plot provided on the diamond certificate offers assurance that the diamond you pay for is the same diamond you receive.
The chart below is a clarity scale showing the different grades of diamond clarity.
Carat
Diamond carat weight is the measurement of how much a diamond weighs.
Each carat can be subdivided into 100 'points.' This allows very precise measurements to the hundredth decimal place. A jeweler may describe the weight of a diamond below one carat by its 'points' alone. For instance, the jeweler may refer to a diamond that weighs 0.25 carats as a 'twenty-five pointer.' Diamond weights greater than one carat are expressed in carats and decimals. A 1.08 carat stone would be described as 'one point oh eight carats.'
All else being equal, diamond price increases with diamond carat weight, because larger diamonds are more rare and more desirable. But two diamonds of equal carat weight can have very different values (and prices) depending on three other factors of the diamond 4Cs.
It's important to remember that a diamond's value is determined using all of the 4Cs, not just carat weight.
Each carat can be subdivided into 100 'points.' This allows very precise measurements to the hundredth decimal place. A jeweler may describe the weight of a diamond below one carat by its 'points' alone. For instance, the jeweler may refer to a diamond that weighs 0.25 carats as a 'twenty-five pointer.' Diamond weights greater than one carat are expressed in carats and decimals. A 1.08 carat stone would be described as 'one point oh eight carats.'
All else being equal, diamond price increases with diamond carat weight, because larger diamonds are more rare and more desirable. But two diamonds of equal carat weight can have very different values (and prices) depending on three other factors of the diamond 4Cs.
It's important to remember that a diamond's value is determined using all of the 4Cs, not just carat weight.