Diamonds are classified according to clarity, cut, color and carat weight, all of which affect their price. Other factors that go into valuing diamonds include polish, proportions, symmetry and fluorescence.
Jewelry professionals grade diamonds systematically, which is how their value is determined. Qualities that are rare in diamonds make diamonds with such characteristics more expensive. For instance, most diamonds have undertones. Therefore, purely colorless diamonds are rare and, thus, pricier. The more the undertones show through, though, the less valuable the diamond.
Clarity relates to whether the diamond has any internal features or external irregularities, called inclusion and blemishes, respectively. Diamonds free of all inclusions and blemishes are rarer and more valuable.
Cut refers to how a rough diamond has been finished. The goal is to cut a diamond to best display its characteristics and luminescence. The brighter the diamond because of its cut, the more valuable it is.
With all the other factors in place, the weight of the diamond determines its price. Jewelers weigh individual diamonds in metric carats. A single metric carat is equivalent to two-tenths of a gram.
Jewelers can get even more specific when grading diamonds, such as how well a stone is polished. They also look at the proportions and symmetry of the cut and how those factor into the diamond's fluorescence.