Carat

The weight or size of a diamond is measured in carats. A one-carat stone is equal in weight to one hundred smaller units called points. Carat weight is the most obvious factor in determining a diamond's value, dependending on the quality of its cut, clarity and color. Be aware that the price of a stone may jump after certain weight thresholds (usually at quarter-carat marks, such as 25 or 75 points).
Approximate sizes as measured in milimeters:

Clarity

Diamonds that are absolutely clear are the most sought-after and therefore the most expensive. But many diamonds have inclusions — scratches, trace minerals or other tiny characteristics that can detract from the pure beauty of the diamond. The GIA and AGSL use a detailed system of rules and standards to summarize the number, location, size, and type of inclusions present in a diamond.

FL, IF Diamonds: Flawless: No internal or external flaws. Internally Flawless: No internal flaws. Very rare and beautiful diamonds.

VVS1, VVS2 Diamonds: Very, Very Slightly Included: Very difficult to see inclusions under 10x magnification. An excellent quality diamond.

VS1, VS2 Diamonds: Very Slightly Included: Inclusions are not typically visible to the unaided eye. Less expensive than the VVS1 or VVS2 grades.

SI1, SI2 Diamonds: Slightly Included: Inclusions are visible under 10x magnification, and may be visible with the unaided eye. A good diamond value.

I1, I2, I3 Diamonds: Included: Blue Nile does not carry diamonds of I-grade clarity.

EXTERNAL FLAWS:
These are any of the impurities that are on the surface of the diamond. External flaws are the result of wear and tear on the diamond or from damage caused during cutting.

INTERNAL FLAWS:
These are any of the many impurities that reside solely in the interior of the diamond. There are many kinds of internal flaws. A few important ones that you should familiarize yourself with are:
§ Black or dark spots: Carbon or other dark spots.
§ Clouds:Clouds are microscopic inclusions that lower transparency.
§ Fracture (feather): The majority of imperfections in diamonds are the result of breakage or fractures, in the diamond chemical structure.

CLARITY GRADING:
The common scale used is based on 10-power magnification (hand held loupe or microscope). This means that for all practical purposes, any imperfection that is too small to be seen with x10 magnification does not "exist" when grading a diamond.
The purer a diamond, the greater its value.

The GIA defines the qualifications for each group in the scale as follows:
Internally flawless (and flawless) group: FL-IF
The highest clarity grade given. “The diamond in question will be without imperfections
visible with x10 magnification. Some blemishes may be permitted externally bringing the stone to a clarity of “IF” or Internally Flawless”.
Very very slightly included group: VVS1 and VVS2
The VVS group is divided into two, VVS1 and VVS2: “The VVS inclusion in question will be very hard to detect, even for an experienced examiner using x10 power magnification”.
Very slight included group: VS1 and VS2
The definition of this group is as follows: “The VS inclusion in question will be hard to detect, even for an experienced examiner using x10 power magnification”.
Slightly included group: SI1, SI2 and SI3
The majority of engagement diamonds bought today are of the SI range: “Inclusions and imperfections in this group may be visible easily with the aid of x10 magnification”. SI3, a relatively new grade added by some laboratories to bridge the very wide gap between SI2 and I1 (imperfect).

SI1: Can be seen easily with x10 magnification. Cannot be viewed with the naked eye.
SI2: Can be seen easily with x10 magnification. Rarely can be viewed with the naked eye.
SI3:Can be seen very easily with x10 magnification. In most cases (stones above 3/4 of a carat) can be viewed with the naked eye.

Color

Acting as a prism, a diamond can divide light into a spectrum of colors and reflect this light as colorful flashes called fire. Just as when looking through colored glass, color in a diamond will act as a filter, and will diminish the spectrum of color emitted. The less color in a diamond, the more colorful the fire, and the better the color grade.

Cut

The cut of a diamond determines its brilliance. There is no single measurement of a diamond that defines its cut, but rather a collection of measurements and observations that determine the relationship between a diamond's light performance, dimensions and finish. Most gemologists consider cut the most important diamond characteristic because even if a diamond has perfect color and clarity, a diamond with a poor cut will have dulled brilliance.

The width and depth can have an effect on how light travels within the diamond, and how it exits in the form of brilliance.


· Ideal cut: Represents roughly the top 3% of diamond quality based on cut. Reflects nearly all light that enters the diamond. An exquisite and rare cut.
· Very good cut: Represents roughly the top 15% of diamond quality based on cut. Reflects nearly as much light as the ideal cut, but for a lower price.
· Good cut: Represents roughly the top 25% of diamond quality based on cut. Reflects most light that enters. Much less expensive than a very good cut.
· Fair cut: Represents roughly the top 35% of diamond quality based on cut. Still a quality diamond, but a fair cut will not be as brilliant as a good cut.
· Poor cut: This includes all diamonds that do not meet the performance standards of a fair cut. These diamonds are generally deep and narrow or shallow and wide and tend to lose most of the light out the sides and bottom. Blue Nile does not carry diamonds with cut grades of poor.

Diamond Cut

As the single human contribution to a polished diamond's beauty, cut is perhaps the most important, yet most over-looked, of the Four Cs of diamond quality. How does cut affect a diamond's value and beauty? A good cut gives a diamond its brilliance, its dispersion, its scintillation-in short, its life. And brilliance is what a diamond is all about, especially in the eyes of the consumer.
The particular angles and finish of any diamond are what determine its ability to handle light, which leads to brilliance. Several factors need to be considered:
The range is:

 Table Diameter: 52.4% to 57.5%

 Crown Angle: 33.7 degrees to 35.8 degrees

 Girdle Thickness: Thin to Slightly Thick (.51% to 2.95%)

 Pavilion Angle: 40.2 degrees to 41.25 degrees

 Culet: None (Pointed) to Medium

 Total Depth: 56.88% to 63.92%