To standardize the diamond industry there are four main attributes on which all diamonds are graded. Cut, Color, Clarity & Carat Weight. Collectively, these attributes are called the "4 C’s" and form the basis for how diamonds are traded and priced.
A. Diamond Shape
There are several diamond shapes that are popular today, with round brilliant diamonds being the most common and having the most sparkle.
All diamonds that aren’t round are categorized as fancy cut diamonds.
Of the fancy cut diamonds, most fall into three cut categories:
Step cut – (Asscher, Emerald, Baguette) Creates a window-like view into the stone. Your eye is drawn into the diamond to show off the stone’s color and clarity.
Modified brilliant cut – (Oval, Marquise, Pear, Heart, Trillion, Cushion) The formula for round brilliant cut diamonds has been adjusted and applied to these fancy shapes to maximize their fire and brilliance.
Mixed cut – (Radiant, Princess) For these shapes, the brilliant cut’s sparkle combines with the step cut’s openness to create the best of both cuts.
A diamond’s cut is made up of two parts: diamond shape and cut grade.
Several factors affect the cut grade of a diamond including its weight relative to its diameter, the girdle thickness, the symmetry of the facets and the quality of its polish.
Well-cut diamonds have facets that optimize the amount of light being reflected into your eye while being ideally proportioned, polished and symmetrical.
Diamonds naturally come in a variety of colors. To the untrained eye, these subtle differences in color are difficult to see. To help measure a diamond’s color, each stone is graded on a scale from D through Z, with D being colorless and Z being light yellow. Colorless diamonds are more rare and as such, command higher prices.
Since diamonds are products of the earth combining intense heat and pressure over hundreds of years, most diamonds have unique marks called inclusions (within the stone) or blemishes (on the surface). The absence of inclusions and blemishes makes a diamond more rare, desirable, and expensive.
Diamonds are graded on a clarity scale from Flawless to Included. The clarity of a stone affects its brilliance, fire and scintillation with some inclusions reducing the amount of light reflected within the stone. Diamonds that are “eye clean” are said to have no inclusions visible to the naked eye.
No inclusions or blemishes are visible to a skilled grader using 10× magnification
Internally Flawless (IF):
No inclusions and only blemishes are visible to a skilled grader using 10x magnification
Very, Very Slightly Included (VVS1, VVS2):
Inclusions are difficult for a skilled grader to see under 10× magnification
Very Slightly Included (VS1, VS2):
Inclusions are clearly visible under 10× magnification but can be characterized as minimal
Slightly Included (SI1, SI2):
Inclusions are noticeable to a skilled grader using 10× magnification
Included (I1, I2, and I3):
Inclusions are obvious under 10× magnification and may affect transparency and brilliance
Diamonds are measured in carats with 100 points per carat, and 1 carat equal to 0.2 grams. (Not to be confused with karat which is a unit used to measure the purity of gold.)
Since diamonds come in various shapes, cuts, clarity and colors, a diamond’s carat weight alone cannot tell you its value. Similarly, looking at the top of a diamond can be deceiving as a basis for estimating a stone’s size. Depending on the cut of the diamond, the stone could appear bigger or smaller than a comparable ideal cut diamond.
Generally, the more a diamond weighs, the more expensive it will be. Larger diamonds are also more rare, which contributes to the higher price tag.
Carat weight of a diamond can be approximated by measuring the distance across the top of a diamond in millimeters and assuming it has a good cut grade.