The Four C's Provide a standard for evaluating diamonds.

Color:  Diamonds can cover the entire spectrum of colors.  The majority, however, range from those with a barely perceptive yellow or brownish tint, up to those that are very rare and described as colorless.  Colorless diamonds are the most desirable since they allow the most refraction of light.  Off-white diamonds absorb light, inhibiting brilliance.  You can observe diamond color by placing the stone table-side down on a flat white surface or grading trough, and examining it from different angles.  Next, place it pavilion-side down with the culet facing you, and examine it through the pavilion facets.

Cut:  The only one of the Four C's affected by intervention, cut has the greatest influence on the diamond's fire and brilliance.  A round, brilliant-cut diamond has 58 facets.  When well proportioned, this shape best shows the stone's brilliance because it allows the most light to be reflected back to the eye of the observer.  Stones that appear lifeless or seem dark in the center are probably poorly cut.  

Clarity:  Most diamonds contain minute traces of non-crystallized carbon and internal stress fractures.  Called inclusions, most of these are not apparent to the naked eye, but can be seen in loose stones under magnification.  Perfect clarity means that no inclusions are discernible when the diamond is examined under a 10X lens.  Inclusions greatly affect the beauty and value of a diamond because they absorb the light rather than allowing it to be reflected back through the front of the stone.  How much the reduce the value of a diamond depends on their size and number, and their position within the stone.  And inclusion in the center of a diamond beneath the table is more visible than one near the edge.  It also may be mirrored many times by adjoining facets.  

FL: Flawless-
No blemishes or inclusions when viewed under 10X magnification.

IF: Internally Flawless-
No inclusions and only insignificant surface blemishes under 10X.

VVS1 and VVS2: Very, Very Slight Inclusions-
Minute inclusions that are difficult to see under 10X.

VS1 and VS2: Very Slight Inclusions-
Minor inclusions ranging from difficult to somewhat easy to see, face up, under 10X.

SI1 and SI2:  Slight Inclusions-
Noticeable inclusions that are easy (SI1) or very easy (SI2) to see under 10X.  Clean face up, to the naked eye

I1, I2 and I3:  Imperfect-
Obvious inclusions that are usually visible, face up, to the naked eye.  Distinctions are based on the combination effect of durability, transparency, and brilliance.

Carat Weight: The term carat comes from the ancient practice of weighing diamonds against the seeds of the carob tree.  The system was eventually standardized and one carat was fixed at 0.2 grams.  One carat is divided into 100 points, so a quarter-carat diamond is 25 points or 0.25 carats.  Although carat is a unit of weight, not size, the carat weight of a diamond has come to refer to particular sizes.  If properly cut, diamonds of the same weight should be about the same size.  There sizes don't apply to other gems, however, because they have different specific gravities than diamonds. 

5 carats= 1 gram