The diamond is among the most coveted of all gemstones. However, all diamonds are not created equally. Every diamond is evaluated using four primary guidelines called the 4 C's.
Diamonds are measured by weight. A one-carat diamond weighs 200 milligrams. You divide each carat for diamonds less than a carat into 100 points; for example, .25 carat equals 25 points. Depending on factors such as depth and proportion, diamonds of equal weight may appear different in size.
You measure the clarity of a diamond by the existence or absence of visible flaws. GIA recognizes 11 grades of clarity, ranging from Internally Flawless (IF) to heavily Included (I3). It is extremely rare to find a diamond that lacks any natural internal or external clarity characteristics.
The Gemological Institute of America (GIA) supports a diamond color scale extending from D (colorless) to Z (light yellow or brown). Although many people think gem-quality diamonds are colorless, a truly colorless diamond is extremely rare -- most diamonds used in jewelry range from near-colorless with slight amounts of yellow or brown. You assign color grades by comparing a diamond to an industry-standard Master Set. The letters indicate how noticeable the color is.
Some diamonds have a degree of fluorescence, a visible light that appears when you expose the stone to ultraviolet radiation, but it is not a factor in determining clarity grades; it is merely a characteristic of the diamond.
The GIA cut scale ranges from Excellent to Poor. There are three factors in determining the quality of the cut. Brightness – the combination of all white light reflected from the surface and the interior of the diamond. Fire – the flashes of color emitted from the diamond. Scintillation – the pattern of light and dark areas and the sparkle you see when moving the diamond. The diamond's proportions affect the light performance, making the diamond beautiful. Diamonds with fine proportions, symmetry, and polish have increased brightness, fire, and scintillation. It takes true artistry by a master craftsman to get the facets proportioned just right to capture light and reflect it back to the eye.