Carat

The carat is a unit of mass equal to 200 mg (0.007055 oz), and it is used for measuring gemstones and pearls. A larger carat means a larger diamond, however since larger diamonds are rarer, a 1-carat diamond is less expensive than 2 1/2-carat diamonds.

Color

Diamonds that enter the Gemological Institute of America's scale are valued according to their clarity and color. For example, a "D" or "E" rated diamond (both grades are considered colorless) is much more valuable than an "R" or "Y" rated diamond (light yellow or brown). This is due to two effects: high-color diamonds are rarer, limiting supply; and the bright white appearance of high-color diamonds is more desired by consumers, increasing demand. Poor color is usually not enough to eliminate the use of diamond as a gemstone: If other gemological characteristics of a stone are good, a low-color diamond can remain more valuable as a gem diamond than an industrial-use diamond, and can see use in diamond jewelry.

Clarity

Diamond clarity is a quality of diamonds relating to the existence and visual appearance of internal characteristics of a diamond called inclusions, and surface defects called blemishes. Inclusions may be crystals of a foreign material or another diamond crystal, or structural imperfections such as tiny cracks that can appear whitish or cloudy. The number, size, color, relative location, orientation, and visibility of inclusions can all affect the relative clarity of a diamond. A clarity grade is assigned based on the overall appearance of the stone under 10x magnification.

Most inclusions present in gem-quality diamonds do not affect the diamonds' performance or structural integrity. However, large clouds can affect a diamond's ability to transmit and scatter light. Large cracks close to or breaking the surface may reduce a diamond's resistance to fracture.

Diamonds with higher clarity grades are more valued, with the exceedingly rare Flawless graded diamond fetching the highest price. Minor inclusions or blemishes are useful, as they can be used as unique identifying marks analogous to fingerprints. In addition, as synthetic diamond technology improves and distinguishing between natural and synthetic diamonds becomes more difficult, inclusions or blemishes can be used as proof of natural origin.

Cut

The cut of the diamond is the most difficult part for a consumer to judge when selecting a good diamond. This is because some certificates will not show the important measurements influencing cut (such as the pavilion angle and crown angle) and will not provide a subjective ranking of how good the cut was. The other 3-Cs can be ranked simply by the rating in each category. It requires a trained eye to judge the quality of a diamond cut, and the task is complicated by the fact that different standards are used in different countries.

Cost

Cost is definitely something to consider while looking at diamonds.