ColorColor is the one of the most important value-setting factors for colored gemstones. All gemstones have what is referred to as a preferred color range. The more the color varies from this range ie: lighter or darker, more vivid or less—the less valuable the stone. Color is composed of three dimensions: hue, tone, and saturation.
Hue refers to the impression of color usually noticed immediately, such as red, yellow or blue.
Tone refers to the degree of lightness or darkness of an object.
Saturation defines the degree of purity of a hue.
CutCut refers to the shape of a stone, its facets, and the precision of the stone's proportions and finishing.
The beauty of a gem is revealed in the cut. Gemstones are most often cut into shapes such as oval, emerald, pear, and round however, they can be carved into almost any design imaginable. Proportions involve the designs balance. Finish refers to the detail and workmanship. You are looking for a stone with a well-proportioned cut and smooth finish. A better cut stone will always be more valuable.
Carat Weight(I had to refer back to the GIA on carat weight I still have a hard time remembering these rules, this data is directly from the GIA website)
The size of a gemstone is measured, not by its dimensions, but by weight. One carat, the traditional unit of measurement for gemstones, is equal to approximately 0.2 grams. You may also hear the weight of a gemstone referred to in points. A point is equal to 1/100 of a carat; therefore a 75-point gemstone equals 0.75 carat. Two different gemstones may have the same dimensions but different weights. This is due to the specific gravity or density of the gem mineral. This difference can help gemologists identify a gemstone.
Up to a certain point, the larger a stone is, the more rare it is and the higher the price it will command. For stones that commonly occur in larger sizes, the value may decrease if the gem reaches a size that makes it impractical for jewelry use.
Source: Gemological Institute of America
ClarityClarity refers to a gemstone’s relative freedom from clarity characteristics. Clarity characteristics include inclusions within the gem, or blemishes found on the surface. The fewer clarity characteristics, the more rare the gemstone. Each variety of gemstone has its own clarity standards. For example, Amber almost always contains clarity characteristics as does Emerald. With the exception of gemstones that traditionally contain clarity characteristics you want a stone that is "eye clean" of inclusions and blemishes this means they can not be easily detected by the naked eye.
There are actually several different clarity types I will discuss theses in a later post.