There are 13 key factors to consider when evaluating a diamond:

However, most individuals can make a reasonable evaluation of a diamond using the 4C's. These consider the weight, shape, color, and clarity of the diamond. The 4C's are:

Carat is a jeweler's term used to describe the size, based on the weight of a diamond. A single carat is a 0.2 metric gram. Assuming that every aspect (or rating) of a diamond was the same except for the carat size, then the larger the diamond the greater the value. Because of the other 3 C's, a larger diamond may not be the more valuable. For example, a diamond with a very poor clarity rating will probably not be worth as much as a diamond with a nearly perfect clarity. Diamonds are also measured in points, with one carat being equal to one hundred points. Normally, the higher the carat weight, the more you pay per carat. For example, two quarter-carat diamonds will not cost the same as a single half-carat diamond of the same quality.

A Clarity rating is used to describe the internal and surface marks of a diamond. The scale ranges from flawless (FL) to imperfect (I) when examined with a 10x magnification. The inclusions in a diamond may be other diamond crystals, small garnets, enclosed fractures or pockets, or other crystals which are common in the diamond. Often inclusions are referred to as flaws, but inclusion is a better term since no naturally created diamond is perfect. Diamonds graded in the I2 or I3 range, may threaten the durability of a diamond. Outside of this range, inclusions are rarely seen by the naked eye. The grading scale, as used by the Gemological Institute of America (GIA) is:

The Color rating is perhaps the most important factor to consider when evaluating a diamond. The scale ranges from colorless (D,E,F) to fancy colors (Z). Colorless diamonds are much more rare than other diamonds and are therefore more expensive.

The general shape and style of a diamond is referred to as its Cut. Different cuts affect the optical properties of a diamond in different ways:

• Brilliance: How "bright" a stone appears. Essentially the reflections of white light within the stone
  
  
• Dispersion: The "fire" of the stone. Dispersion is what causes a prism effect in a diamond.
  
  
• Scintillation: How a stone "sparkles". These are the flashes of light from the facets of a gem when you move the stone.
  
  
• Luster: The "shine" of the stone. How much light is reflected from the stone.