If you are planning on buying a diamond engagement ring for your girlfriend, then you need to brush up on some of the technical jargon that diamond industry professionals use. This way you will not be intimidated when it comes time to buy a ring. The terms listed below are two of commonly mentioned to 4c's in the diamond world. The other two not covered are color and carat (which are simple enough to understand).
The following terms are used to describe the more difficult to understand aspects of a diamond. Once you have a brief understanding of them, you will be much better prepared to choose a diamond ring.
Diamonds are taken rough from the earth and then cut to shape. You can choose any design you think your girlfriend would like (or that she has mentioned she prefers) such as a princess cut, an asscher cut, or a pillow. However, the term "cut" refers to how well the chosen design was cut into the stone, regardless of which design was used.
It takes into account if the angles are properly aligned, if the pavilion is well seated, if the stone has too shallow a crown, and other issues. Everything has to do with balance and symmetry. There are terms used when discussing the cut (pavilion, crown, culet, girdle) that are used to describe different aspects of the diamond body, but the main thing you want to know is if the "cut" is good. Luckily for the non-expert, the best way to do this is to choose a diamond that has been graded by one of the popular agencies. They will have examined the stone and determined if the angles are proper.
One of the reasons the cut is so important is that it can effect the way light bounces off the diamond. While you might not notice a crooked cut, it can effect the "fire" or "brilliance" (terms that deal with the way a diamond shines and reflects light). So, it is important to choose a well cut diamond because it will shine on her finger.
Clarity refers to how few visual disturbances the diamond has in the form of cracks, inclusions, or other blemishes. All diamonds will have some sort of blemish. The important aspect of the diamond choosing process is to pick a diamond with as few inclusions as you can afford. These small blemishes can interfere with light passing through the stone, and thereby dull its shine.
Now, some cracks or surface blemishes will be obvious to the naked eye, but really damaged diamonds tend to be used for industrial purposes. When looking at a diamond ring, you will mostly be dealing with issues that are seen through a 10 times magnifier (the jewelers loupe). The better the clarity, the higher grade of diamond. Again, if you are not comfortable examining the diamond yourself, it's fine because you can just rely on the diamond grade.