Colored gemstones are evaluated for their cut, color, clarity, and carat weight -- just as diamonds are -- however, with different approaches. For example, color is most important. The color of ruby, for instance, cannot be so light as to appear pink. It must have enough saturation and tone to appear red. The color is not usually just red either. It typically is purplish red or orangy red. Then, if the tone is too dark, it loses its red and takes on a blackish look.
The clarity is evaluated by the look of the gem with the unaided eye, whereas diamonds are evaluated using 10-power magnification. Not all visible characteristics are considered detrimental to the value of the gemstone.