Cut is perhaps the most important of the four Cs, so it is important to understand how this quality affects the properties and values of a diamond.

The angles and finish of any diamond are what determine its ability to handle light. A good cut gives a diamond brilliance (the brightness that seems to come from the very heart of the diamond) and scintillation and dispersion (the sparks of color that seem to leap out from the diamond's surface when the diamond is tilted back and forth).
As you can see in the images below, when a diamond is well-cut, the light that enters through the table travels deep into the pavilion. Once light reaches the pavilion, it bounces back and forth on the mirror-like inside walls of the pavilion, multiplying in brightness and intensity before reflecting back out of the diamond through the table and to the observer's eye.

To understand how the pavilion works to make a diamond so brilliant, think of an empty room that has a single candle as its only source of light. If a mirror is added to the room, the mirror will reflect the image of the candle, creating a second image of the flame and essentially doubling the amount of light in the room. If more mirrors are added to the room, the brightness in the room will increase as the number of reflections of the original candle flame increases. A well-cut diamond works in much the same way as a series of mirrors that multiply the brilliance of the original light source. This light is the brilliance we mentioned, and it's this effect that makes diamonds so mesmerizing.

However, when a diamond is poorly cut (either too shallow or too deep), the light that enters through the table reaches the pavilion facets at the wrong angle and fails to be intensified and reflected properly; it 'leaks' out from the sides or bottom of the diamond rather than reflecting back through the table to the eye. Less light reflected back to the eye means less brilliance.

Clarity:

Clarity ratings are determined by the number of inclusions (tiny pinpoints, markings, clouding, etc.) and how visible they are. Diamonds with fewer inclusions have better clarity, which increases their brilliance and value. Diamonds with clarity grades of FL and IF contain no internal inclusions that are visible under 10x magnification. Diamonds with clarity grades VVS1 through SI2 have inclusions that are visible to varying degrees under 10x magnification, but should not generally be visible to the unaided eye. Diamonds with I1 clarity contain inclusions that are easily seen under 10x magnification and may be visible with the unaided eye. Diamonds with I2 and I3 clarities have major inclusions that are easily seen with the unaided eye.

FL – IF: Flawless, Internally Flawless. A clarity grade of FL means that no inclusions or surface blemishes can be seen under 10x magnification. A diamond with a clarity grade of IF (Internally Flawless) has no inclusions that are visible under 10x magnification.

VVS1-2: Very, Very Slightly Included. Diamonds with a VVS1 or VVS2 clarity grade have tiny inclusions that are very difficult to see under 10x magnification.

VS1-2: Very Slightly Included. Diamonds with a VS1 or VS2 clarity grade have small inclusions, such as pinpoints, included crystals, and feathers, that are visible with some difficulty under 10x magnification. Diamonds with this grade offer beautiful clarity at a lower cost than VVS or IF diamonds.

SI1-2: Slightly Included. Diamonds with a SI1 or SI2 clarity grade have small inclusions, such as pinpoints, included crystals, knots, clouds, and feathers, that can be seen somewhat easily under 10x magnification. It should be difficult to see any internal inclusions with the unaided eye. Diamonds with this grade should cost less than diamonds with a VS or VVS rating.

I1: Included to the first degree. Diamonds with an I1 clarity grade have inclusions that are numerous or large enough to very easily be seen under 10x magnification, and may even be visible to the unaided eye if the location of the inclusion is known to the viewer. Diamonds of this clarity generally cost much less than diamonds with an SI clarity grade.

I2-I3: Included to the second and third degree. Diamonds with an I2 or I3 clarity grade have major inclusions that are easily seen with the unaided eye. Diamonds with these clarity grades can have inclusions that are numerous and significant enough to affect the transparency and brilliance of the diamond.

Color:

Diamonds with an absence of body color most purely reflect the full color spectrum and are therefore the most valuable. Most diamonds come in a wide range of colors and are graded on a letter scale from D (colorless), the highest grade, through Z (usually a light yellow, brown, or gray). It is difficult for the untrained eye to see variations in color grades D through J unless stones are being compared side by side.

Diamonds should be color-graded under exacting conditions by an expert grader using specially filtered cool white light. They should be compared to a set of diamonds with known colors, graded by the Gemological Institute of America against their master set. To obtain the most accurate color grade, graders usually place diamonds on their sides or upside-down against a neutral background, to help reduce the play of spectral colors that diamonds reflect.

Carat Weight

A carat is the unit of weight by which a diamond is measured. Because large diamonds are found less commonly than small diamonds, the price of a diamond rises exponentionaly to its size.