The "four C's" form the basis for evaluating a diamond and determining it's value.
Color, Clarity, Cut and Carat are the four aspects involved. Each is important and impacts the appearance of the stone in a different way. With a basic understanding of these factors it will be possible for you to determine the combination that delivers the best value for you.

Color
The absence of color allows the most natural light to pass through the diamond. Therefore, for most preferences, the best color is no color at all, or "colorless". Most diamonds have a lesser or greater degree of yellow or brown color, sometimes so slight that it is virtually imperceptible, while many others on the market display more readily visible tints. Some rare diamonds have very intense or exotic colors (canary, pink, blue, green, purple, or even red) and can be extraordinarily valuable.

Scale
A diamond's color is graded on an alphabetical scale from D-Z, with D being absolutely colorless and Z being light yellow. Beyond "Z", a diamond is considered to be an exotic or "Fancy" color. Although many diamonds appear to be colorless, the vast majority have at least a hint of body color.

If all other factors are equal, the less color in a diamond or the higher color rating, the more valuable a diamond becomes. Likewise, as the amount of color increases, the price of a diamond decreases (though this does not necessarily reduce the beauty of a diamond.)

Florescence
Fluorescence, also known as photoluminescence, is a property in some diamonds that makes them glow a certain color (usually blue) when placed in an ultraviolet rich environment. Strong, very strong and sometimes medium blue fluorescence will tend to neutralize any yellowish or brownish tint in diamonds 'H' in color or below (I,J,K etc) giving them a whiter appearance. Such fluorescence in some of the 'lower' color grades may even enhance the value of these stones. Strong blue fluorescence in diamonds will generally not improve the look of a stone D to F in color may even detract a few percent from the value of those diamonds. On rare occasions, some diamonds with very strong blue fluorescence can have a hazy or 'milky' appearance that significantly reduces their value. We always attempt to avoid offering such stones for sale.

Impact on Beauty and Price
Impact on Beauty: Diamond color grades D through J are considered colorless or nearly colorless, and as such will make exquisite jewelry. D, E, and F colors, however are the most highly prized as they are considered "colorless." Please keep in mind that earrings and pendants do not generally receive the same scrutiny as the feature diamond in a ring. For that reason, you might consider sacrificing a little in color in exchange for greater size.
Impact on Price: Subtle differences in color may account for dramatic differences in price. When you are certain of the color grade of the diamond (per its certificate and/or independent appraisal), selecting a diamond that is a grade or two lower than another will reduce the cost per carat and may allow for the selection of a larger diamond - often with little if any visible difference when the stone is mounted.

Clarity
Clarity is an indication of a diamond's purity. It describes quite literally the degree to which a diamond is free of imperfections. The clarity grades range from internally flawless (IF) to highly imperfect (I3).

Flaws in diamonds include "blemishes" and "inclusions." Blemishes are an exterior imperfection on a diamond, usually a feature of the cutting or polishing of the stone. Most blemishes are so small as to have no affect on the beauty or brilliance of the stone. Inclusions are tiny, usually microscopic, imperfections inside a diamond. In all diamonds, except the most rare, traces of minerals, gasses or other elements were trapped inside during the crystallization process. Inclusions may look like tiny crystals, clouds, or feathers and are unique to every diamond. Many of these birthmarks are not visible to the naked eye. In fact, it is very rare to find a diamond that is completely clean to the expert eye using magnification.

The clarity of a diamond is graded by how many, how big (relative to the size of the stone) and how visible the inclusions are, and where they are located within the diamond. The fewer and smaller the inclusions, the more rare and valuable the diamond. Less than 1% of all diamonds ever found has had no visible inclusions and can be called internally flawless (IF).

Abbreviations
A diamond's clarity grade is described using the following universally accepted abbreviations.

Abbreviation Definition Description F, IF Flawless - Internally Flawless stones have no external blemishes or internal inclusions when examined by an experienced grader using 10X magnification. Internally Flawless diamonds have no inclusions under 10X magnification but may have some minute external blemishes. VVS1, VVS2 Very, Very Slightly Included Contains minute inclusions that are difficult for even experienced graders to see under 10X magnification. VS1, VS2 Very Slightly Included Clean to the naked eye; contains minute inclusions when examined under 10X magnification. SI1, SI2, SI3* Slightly Included Ranges from clean to nearly clean to the naked eye (eye clean) when viewed from the top. I1, I2, I3 Included Ranges from being nearly eye clean to having very easy to find imperfections to the naked eye which might affect the brilliance of the stone.

Impact on Beauty and Price
Impact on Beauty: The clarity grade of a diamond may have little or no effect on the beauty of a diamond if the clarity grade is within the first seven or eight grades (Flawless-SI1 or SI2.) Most people can't much visible difference among stones until reaching the "imperfect" grades. Please keep in mind that earrings and pendants do not generally receive the same scrutiny as the feature diamond in a ring. For that reason, you might consider sacrificing a little in clarity in exchange for greater size.
Impact on Price: As in color, small differences in clarity can have a great impact on price. If beauty is the chief concern, it is advisable to stick with a well made diamond of SI1-2 clarity or better. Within your budget, try to arrive at a good balance between overall quality and size.

Cut
Cut actually refers to two aspects of a diamond. The first is its shape (round, marquise, etc) which is explained below, and the second is how well the cutting has been executed which we will discuss here. The cut or "make" of a stone is one of the most important of all diamond characteristics, and among the hardest to judge. The proportions of a stone as well as its polish and precision of faceting determine how much of the diamond's potential fire and beauty may be released.

Diamond cutters are paid to maintain a balance between retaining the maximum weight from rough stones and cutting the finest possible diamond at the expense of "yield." Some diamonds are cut exceedingly deep or shallow, or are shaped to conform to the original shape of the rough stone. A poorly made stone tends to result in a higher yield (less waste) from the rough while a better made diamond "wastes" more of the rough. A well-cut round diamond typically weighs only about 40% or less of the original weight of the piece of rough the cutter started with. This is why better cut diamonds command a premium, and why Ideal cut round diamonds are the most valuable of all.

Ideal Cut Diamonds
An ideal cut has the highest quality cut proportions available to diamonds. It should also be noted that the "ideal" and "premium" grades only apply to round brilliant cuts. There is considerably less agreement about the best combination of proportions necessary for optimum performance in fancy shapes.

The way a diamond is cut profoundly influences its sparkle, fire and brilliance, as well as its perceived size and even, to some degree its apparent color. In order to maximize the diamond's brilliance it must be well polished and cut in a geometrically precise manner. This means properly aligning the facets so light will enter the diamond and reflect back through the large top facet, or table of the diamond.

The most basic measurements of a diamond, expressed in millimeters actually describe the following: minimum width, maximum width (or length), and depth. These measurements are important in matching stones for use in earrings and other jewelry and for evaluating the cut quality of a stone. All other features describing the cut of a diamond, with the exception of polish, are based on the stone's measurements.

Diamond Anatomy
Start by learning the "anatomy" of a diamond. The proportions of these are critical measures in determining the quality of a diamond's cut.

Facet:
The polished planes (surfaces) on the surface of the diamond, defining its dimensions.

Crown:
The faceted portion of the diamond that is above the girdle. The crown is generally the portion of the diamond that is visible above the setting.

Table:
The table is the top and largest facet of a polished diamond through which much of the light both enters and exits.

Girdle:
The outermost edge of a diamond; it can be unpolished or polished and faceted and of varying thickness. The girdle is generally where the diamond is held in a setting. Girdles may be laser-inscribed with serial numbers, names, etc. without affecting the diamond's overall brilliance or value.

Pavilion:
The portion of the diamond that is below the girdle.

Culet:
The small facet or point at the bottom of the pavilion of a diamond.

Polish/Symmetry
Although most non-experts could never notice subtle differences in polish or symmetry, noticeably asymmetrical stones denoting poor symmetry are less attractive and less valuable - just as a poor polish may detract from the brilliance of a diamond and may actually leave slight streaks on the surface. Below are some cut characteristics, and the effect they have on the overall quality of the diamond.

Symmetry:
Symmetry refers to the overall uniformity of the cut of a diamond. Symmetry is based on the diamond's proportions, the relation of one facet to another. A diamond with good symmetry can be more attractive than a less symmetrical stone. Stones with poor symmetry can actually appear off-centered or noticeably out of round.

Polish:
The overall quality and uniformity of the polish of a diamond. Its grade is based on the final finish applied to the facets and facet junctures by the cutter. Well-polished diamonds permit maximum passage of light and prevent potentially streaky surfaces.

Table/Table Percentage:
The table is the top-most and generally the largest facet of a diamond through which much of the light both enters and exits. A table that is too large or too small will reduce the overall dispersion of a diamond's brilliance. In combination with other factors, the table percentage is a significant component of a diamond's overall cut quality and value

Depth/Depth Percentage:
The depth is a measurement of the distance from the table to the culet (bottom-most point). A depth that is too shallow or too deep will allow light to escape through the bottom of the stone, reducing the stone's overall fire and brilliance. Depth percentage is a significant component of the diamonds overall cut quality and value.

Crown/Crown Angle:
The crown is the top portion of the diamond that lies above the girdle. Crown % is a measurement that relates the depth of the crown to the diamond's overall depth.

Girdle:
The outermost edge of a diamond; it can be unpolished or polished and faceted and of varying thickness. Extremely thin or extremely thick girdles are less desirable than more moderate girdles. The girdle is generally where the diamond is held in a setting. Girdles may be laser-inscribed with serial numbers, names, etc. without affecting the diamond's overall brilliance or value.

Culet:
The bottom-most facet or point of a diamond. Unless the culet is abnormally large, it generally has no impact on the value of a diamond.

Length/Width Ratio:
Applicable for fancy shapes (non-round). As you look down at the stone, the length/width ratio describes the shape of the profile. Although some ratios are generally viewed as more desirable than others for each fancy shape, the length/width ratio that is right for you is a matter of personal preference.

Carat
Diamonds are sold by the carat (ct), not to be confused with karat (kt), which refers to gold purity. Jewelers often refer to the carat weight of diamonds in terms of percentage points. This is particularly true of stones under one carat. There are 100 points to a carat, so if a diamond weighs 50 points, it is .50 of a carat.