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Color
The color of a gemstone is graded on hue, saturation and tone. Hue refers to the color or shade, such as red, green, yellow, or blue. Saturation is a reference to the strength of the color. Tone measures the lightness or darkness of the color. In general, stones receiving the highest marks are pure in color and rich in hue. For example, a greenish-blue gem will not be graded as high as a gem which is pure green. The color ratings are adjusted for certain gems. For example, diamonds are rated on their lack of color, with colorless diamonds receiving the highest grade. Some gems come in a variety of colors, and therefore will be measured more strongly on their other values.
Clarity
Clarity is a measure of inclusions within a gemstone. Inclusions refer to traces of outside particles or minerals present in the gemstone. When grading clarity, the stone is compared to the normal standard of that particular gem. A gemstone may receive the grade of "eye clean," meaning the inclusions are not visible to the naked eye, or grades of slightly included, moderately included or heavily included. The GIA uses a more precise system of grading for clarity, ranging from VVS, which is very, very slightly included, to Dcl, which stands for declasse. Any gem hoping to receive an overall A+ grade must score a VVS on this scale, with exception to some stones which simply do not occur in nature at this level of clarity.
Cut
The cut of a gemstone is one of the most important factors effecting its appearance and overall grading. Most gemstones are found in their rough, natural form. The art of cutting the stone, or "lapidary," is a precise science. There are four main ways to shape a gemstone. Tumbling involves a high pressured polishing of the stone. The stone is not altered greatly in shape, however, it is smoothed overall. Cabochon cutting produces a stone with a flat bottom and a dome-like top. Faceting produces a gemstone with many geometrically arranged flat surfaces with most diamonds are cut in this way. Carving is the most difficult lapidary art, producing intricate shapes and designs on the gemstone. When grading the cut of a gemstone, the method used will be considered along with several other factors. Symmetry of the stone is measured, along with reflection of light. There is not a universal grading method for cutting, however, a gemstone graded overall as an A+ will have a superior cut, regardless of the cutting method employed.
Carat
Gemstone carat is a measurement of the weight of the stone. One carat is equal to 200 milligrams. Carats are further divided into a point system for more accurate rating, with 100 points assigned to each carat. Typically, the larger the carat, the greater the grade. When grading carat, the gem is compared to other sizes of the same gem. An A+ gemstone will receive the highest marks for carat in its particular gem category.