4 Cs of Diamonds

Cut, Clarity, Color and Carat Weight

Shape refers to the general outward appearance of the diamond, (such as round, emerald, or pear). When a diamond jeweler (or a diamond certificate) says "cut," that's a reference to the diamond's reflective qualities, not the shape.
diamond cut information diamond clarity information diamond color information diamond carat information
CUT: The shape of a diamond is commonly referred to as the cut - For example:

The shape of a diamond and it's cut are commonly confused, as a diamond is available in fancy shapes as well.

The diamond's cut is really the craftsmanship applied in cutting the facets of the stone. The artisan attempts to cut a diamond to make the best use of light. When a diamond cut is to good proportions, light is refracted from one facet to another, then dispersed through the top of the diamond. When a diamond is cut too deep, some light escapes through the opposite side of the diamond. If the diamond is cut too shallow, light escapes through the bottom of the stone before it can be reflected.

In addition, the facets must be placed with precise symmetry for maximum brilliance. The quality of the "cut" does make a difference in how a diamond looks.
CLARITY: Clarity is an indication of a diamond's purity. When a rough stone is extracted from carbon, deep beneath the earth, tiny traces of natural elements are almost always trapped inside.

These elements are called inclusions, though sometimes referred to as birthmarks, because they are formed naturally and are unique to each stone.

Inclusions include flaws such as air bubbles, cracks, and non-diamond minerals found in the diamond. Blemishes include scratches, pits, and chips. Some blemishes occur during the cutting processes (most often at the girdle). Diamonds with no or few inclusions and blemishes are more highly valued than those with less clarity because they are rarer.

It takes pressure to create a natural diamond. Genuine (certified) diamonds are not grown in a laboratory, so it's natural for most diamonds to have flaws. Because most inclusions are not visible to the naked eye, a jeweler will use a magnifier known as a loupe to reveal a diamond's inclusions.
COLOR: Color is the natural body color visible in a diamond and is the one C determined completely by nature, not man. As a rule, the closer a diamond is to colorless, the more valuable and beautiful it is. Diamond colors appear in a range. Color grades D, E and F are considered colorless, making them rare.
CARAT: A diamond's size is measured in carat weight. Each diamond carat is also equal to 100 points. For example, a diamond that is a 1/2 carat can also be referred to as a 50-point diamond. But keep this in mind: Bigger isn't necessarily better. A two-carat diamond that is cut poorly is not nearly as beautiful as a smaller diamond that's cut by a skilled diamond cutter. Or, it may be cut well, but have poor color and clarity.