Cut:

There is Cut. And then there is Cut.
Many people confuse cut with the shape of a diamond. Diamonds are cut into a number of shapes, depending on the rough stone. The most popular are round, marquise, oval, pear, heart, and emerald, and the choice is largely a matter of personal prefernece. Whatever the shape, however, a well-cut diamond is the work of a master diamond cutter. When cut to good proportions, the diamond is better able to handle light, creating more scintillation, more sparkle. And that's a great reflection on you.

Color:

Good color is no Color.
Diamonds are found with a range of colors, from faint yellow or brown through to the very rare pinks, blues, greens and other colors known as "fancies". However, the best color for a diamond is no color. It is a totally colorless diamond that allows white light to pass through it and be dispersed as rainbows of color. And to give a woman such a diamond is to show your true colors, as well.
Color grading scale from totally colorless to light yellow. The differences between one grade and another are very subtle, as can be seen by the number of grades within any one category.

Clarity:

A rarity.
To determine a diamond's clarity, it is viewed under 10-power magnification by a trained eye. Most diamonds contain very tiny natural birthmarks known as inclusiions. However, the fewer and smaller the inclusions are, the less likely it is that they will interfere with the passage of light through the diamond, and the more beautiful the diamond will be. Diamonds, more that any other gemstone, have the capability to produce the maximum amount of brilliance. A diamond that is free of both inclusions and surface blemishes is very rare and therefore more valuable. So, the clearer the diamond, the clearer your choice.

Carat-Weight:

Bigger is Bigger (and not necessarily better).
This is the weight of the diamond measured in carats. One carat is divided into 100 "points", so that a diamond of 75 points weighs .75 carats. Carat-weight is the easiest of the 4Cs to determine. But two diamonds of equal weight can have very unequal value, depending on their cut, color and clarity. Fine quality can be found in diamonds of all shapes and sizes. And a fine quality diamond is a little extra weight she'd be pleased to put on.