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

Cut:
Cut has the greatest influence on the diamonds fire and brilliance.  A round, brilliant-cut diamond has 28 facets.  When well proportioned, this shape best shows the stones brilliance because it allows the most light to be refracted back to the eye of the observer.  Stones that appear lifeless or stones that appear dark in the center are poorly cut.  When the angle relationship between the crown and pavilion facets is correct, rays of light entering the diamond strike the rear facets at an angle greater than the critical angle. (24.5 degrees for diamond), and reflect back to the eye of the observer.  If the stone is cut too deeply the light strikes the rear facets at an angle less than the critical angle and the light is lost through the sides of the diamond.  If the diamond is cut too shallow, the light passes through the diamond without being reflected back. (no sparkle).

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

Carat Weight:
The term "CARAT" comes from the ancient practice of weighing diamonds against the seeds of a 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 carat.  Although carat is a unite 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.  These sizes do not apply to other gems, however, because their specific gravity's differ from a diamond.