Diamonds: the 4 C's
Cut: The proportions to which a diamond is cut, the crown height and angle, the pavilion depth and angle, the table size, the culet, the girdle, the quality of polishing, the symmetry of execution. All of these to a Gemologist tell the quality of the cut. To the consumer, they see a heightened sparkle called scintalation.
Carat Weight: Every diamond in its loose unset form is placed on a scale and weighed in carats. If cut properly, it will correlate to a measured size, or what we see as the size.
Clarity: Diamonds are pure carbon, impurities can show up in many different forms in the crystalline structure, most diamonds have them somewhere. The placement and the visibility of these "inclusions" determine its clarity grade, a letter grade.
Color: Every diamond is compared to a "Master Set" of diamonds, calibrated to a color scale starting a D and ending at Z (designated for fancy colored diamonds). Complete lack of color is ideal and graded D, it should look like a drop of water with no tonings of yellow, silver, or brown. The degree to which a diamond possesses a hue correlates to a letter on the scale. DEF is colorless, GHI is near colorless. JKL will show color coming through the top side of the diamond. All color grades are assigned looking through the side and bottom of the diamond, turning away the reflected light from the faceting.