The GIA (Gemological Institute of America) rates diamonds on four main characteristics. These characteristics are defined under "The 4 C's"
Clarity
Clarity grades are based on the number and location of naturally occurring inclusions within a diamond as seen with no more than 10x magnification.
The most common inclusions are carbon (black spots), feathers & clouds (white areas), and cleavages (small ”cracks”).
Any diamond graded SI2 or above will not have inclusions that you can see with the naked eye. Price increases dramatically with a higher clarity grade.
Cut
A fine cut or ”make” can make an enormous difference in the sparkle and brilliance of a diamond. Round diamonds usually have 58 facets and are called brilliant rounds. The crown facets diffuse light into “fire,” while the table width and other proportions determine the brilliance.
​
The cut is crucial to the quality of the diamond, because of its influence on the refraction of light entering the stone. Most of the light entering the top of the stone needs to be returned back in order for it to “sparkle.” If the stone is cut well it will return most of the light, revealing the brilliance and sparkle of the diamond.
​
When discussing the purchase of a diamond with your jeweler, it is helpful to know the 'anatomy' of a diamond. Each surface of a diamond has its own name and singular importance to the overall appearance and beauty each diamond possesses.
Color
The diamond color evaluation of most gem-quality diamonds is based on the absence of color. A chemically pure and structurally perfect diamond has no hue, like a drop of pure water, and consequently, a higher value. GIA’s D-to-Z diamond color-grading system measures the degree of colorlessness by comparing a stone under controlled lighting and precise viewing conditions to master stones of established color value. Many of these diamond color distinctions are so subtle that they are invisible to the untrained eye; however, these distinctions make a very big difference in diamond quality and price.
Carat
Diamond carat weight is the measurement of how much a diamond weighs. A metric “carat” is defined as 200 milligrams. 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 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's 4Cs: Clarity, Color, and Cut.
It’s important to remember that a diamond’s value is determined using all of the 4Cs, not just carat weight.
