What are the 4 C's?
1. Cut: The most important diamond characteristic. A diamond's cut determines how much it sparkles. It's all about light: how much is taken in, how it's refracted around inside and how much is reflected back to the eye of the beholder. The number of facets, the angle of the facets, and the symmetry and alignment of the shape will affect how the diamond returns light. There is more brilliance if the diamond is not cut too deep or too shallow. The better the cut, the more sparkle!
2. Color: A diamond's color grade is based on lack of color. Although most diamonds may appear colorless, they actually have subtle shades of yellow or brown. The closer a diamond is to having no color, the rarer and more expensive it is. Less color = higher the grade. 
3. Clarity: Diamond clarity is the assessment of small imperfections on the surface and within the stone. Surface flaws are called blemishes, while internal defects are known as inclusions. In most cases, a diamond's beauty is not affected by these in any way since most inclusions can't be seen with the naked eye. Remember all diamonds are unique, not perfect. They are made underground through enormous pressure and heat. Natural inclusions and blemishes are inevitable. Flawless diamonds are very rare and also very expensive. We do not recommend buying a flawless diamond.
4. Carat: Remember that carat is the weight of the diamond, not the size. A 1 carat well cut diamond can look larger than a 1 carat poorly cut diamond. Bigger is not always better. Remember that not all diamonds look the same so it is better to see different diamonds in person to compare their carat sizes, even if they are all the same weight.
Below is a diagram of a diamond.
