Like diamonds, colored gemstones are evaluated according to the 4 Cs — color, cut, clarity, and carat weight. Some gemstones come in a wide variety of colors, while others have more uniform palettes. Each type of gemstone is judged by its own potential.
There are three aspects used to describe color: hue, saturation, and tone. Hue is the shade of a particular color -- red, green, blue, and so on. Saturation measures the richness and the purity of the color. Tone is how light or dark the color is. Generally speaking, gemstones of pure hue, medium tone, and high saturation are the most desirable and valuable.
The cut of a gemstone is the way it's shaped and the configuration of its facets. Gemstones usually come in a familiar selection of shapes -- round, oval, cushion, emerald, and pear -- but can be cut into a wide variety of designs. A stone can be cut with facets, like a diamond, or have a smooth shape, which is called a cabochon. A spectacular cut increases the stone's value dramatically.
A gemstone's clarity describes its natural internal characteristics or inclusions. Some gemstones have more inclusions than others, while others have few or none. Emeralds and sapphires, for example, are very valuable gemstones that have inclusions. Aquamarine, citrine, tanzanite, and topaz are examples of stones that have few visible inclusions. Each type of gemstone has a defined clarity standard which differs from stone to stone.
The size of a gemstone is measured by its weight in carats. One carat is equal to one-fifth of a gram (0.2g). Gemstones are measured by their weight, because each stone has a different density. Denser stones look smaller at the same weight as less dense ones. Usually, the heavier a gemstone is, the more it's worth.