Measured in carats, named after the carob seed it was originally used for, 1.00 carats is equal to 0.20 grams. To understand carats, consider the similarities to dollars and cents.
0.50 carats = 1/2 carat
0.25 carats = 1/4 carat
Cut - Scale
Not to be confused with shape, the cut scale applies to round diamonds only...PERIOD. Do not be fooled by fancy cuts with "Ideal Cuts", there is no such thing. All other factors equal, an excellent cut diamond will sparkle more than a poorly cut diamond.
Cut - Shape
There are over 20 different shapes of diamonds if you include accent shapes. However, the ten most common are pictured left. Round has been, and will likely remain, the most popular. Thus, prices for rounds are slightly higher than all other shapes.
The term used for grading internal and/or external flaws. The naturally occurring internal varieties and naturally or accidental (i.e. chips) external varieties affect a diamond's value. Most important take away...there is a range within each clarity grade. Not all SI1's are created equal.
Perhaps the easiest "C" to grasp; is the diamond white or yellow? But as in life, nothing is black and white. These are the 7 color grades that are considered to be a shade of colorless (DEF=colorless; GHIJ=near colorless)