Diamond carat weight is the measurement of how much a diamond weighs. A metric "carat" is a small unit of measurement equal to 200 milligrams. Each carat can be subdivided into 100 points. A .75 carat diamond is the same as 75 points or 3/4 carat diamond. 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: Color, Clarity and Cut. The diagram to the right shows the size of various carat weights of a diamond in relation to each other.
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 masterstones stones of established color value.
Diamonds contain unique birthmarks, either internal (inclusions) or external (blemishes) because of the extreme heat and pressure they face when formed deep within the earth. Clarity refers to the absence or presence of these inclusions and blemishes. Many inclusions and blemishes are too tiny to be seen by the naked eye. A 10x magnification loop is used to view inclusions. Where inclusions are located on a diamond is extremely important and can greatly affect its value. Inclusions off to the side, ones that can be hidden by a prong are more desirable than inclusions in the middle or top of a diamond that could impact the brilliance of the stone. Using the GIA International Diamond Grading System™, diamonds are assigned a clarity grade as shown below:
Diamonds are renowned for their ability to transmit light and sparkle so intensely. A diamond's cut grade is about how well a diamond's facets interact with light. Precise artistry and workmanship are required to fashion a stone so its proportions, symmetry, and polish deliver the magnificent return of light only possible in a diamond. A diamond's cut is crucial to the stone's final beauty and value. And of all the diamond 4Cs, it is the most complex and technically difficult to analyze.
To determine the cut grade of the standard round brilliant diamond - the shape that dominates the majority of diamond jewelry – GIA calculates the proportions of those facets that influence the diamond's face-up appearance. These proportions allow GIA to evaluate how successfully a diamond interacts with light to create desirable visual effects such as:
Brightness: Internal and external white light reflected from a diamond
Fire: The scattering of white light into all the colors of the rainbow
Scintillation: The amount of sparkle a diamond produces, and the pattern of light and dark areas caused by reflections within the diamond