Since small differences in the carat weight can cause a huge price difference, measuring is conducted with great precision: Carat weight is often measured to the hundred-thousandths of a carat. However, you’ll find the carat weight declaration rounded to the hundredth of a carat on most diamond certificates.
Although Carat might be the most commonly known characteristic of diamonds, the value and quality of a diamond also depend on the other Cs:
When it comes to cut, you need to differentiate between the shape or cutting style and the cut grade of the diamond: Shape and cutting style as shown on a GIA certificate describe the actual form a diamond was cut into. The cut grade measures the craftsmanship quality of the cutting process. It’s the cut quality that gives a diamond it’s sparkle and brilliance: The appearance of a diamond largely depends on the quality of its cut.
The scale assessing the cut on a GIA certificate ranges from Excellent to Poor. Each one of these grades represents a whole range of proportion sets and face-up appearances.
Diamonds consist of only a single element, which is carbon. Intense heat and pressure over a long time turn the carbon molecules into a diamond. However, every diamond contains unique internal or external “birthmarks”, called inclusions (internal) and blemishes (external).
It is the inclusions and blemishes that make a diamond truly unique and unmistakable. Some diamonds come close to perfect, but most jewelers have never seen one.
The diamond's clarity, which measures the absence of inclusions and blemishes, is described by terms like FL (Flawless) for diamonds with no visible inclusions under 10x magnification, VVS for very slightly included or I for inclusions that are visible under the magnifying glass and may affect transparency and brilliance.
Color measures how white a diamond is. D-colored stones are absolutely colorless, which is what is most sought after. On the lower end of the scale, diamonds with a Z color grade have a light yellow or brown tint. Each grade in-between D and Z stands for slightly different shade. While the differences between them are often hard to see for the untrained eye, they still contribute to the value of a diamond.