The value of your diamond is determined by four characteristics, in the industry this is known as The 4 C's. It is also recognised in the industry that there may well be a fifth "C" which also assists when selling the diamond. These characteristics are:
The Cut of a Diamond should not be confused with the shape. The Cut of the Diamond refers to its reflective qualities.
When a diamond is cut poorly the light will not create less reflection and sparkle, it loses this value when it is poorly cut. A Diamond cut ideally will reflect more of the light directly and create that magical reflection which a Diamond is famous for.
In the same fashion as the Cut of the Diamond, a Diamond that has more clarity will also create a finer visual experience, a sparkle that diamonds with flaws just can't compete with.
There are many different colours of diamonds but these are classified as the grade, most of which are based off white and moving towards yellow. The most expensive Diamond will be categorised as "Exceptional White", moving towards "Rare White", "Tinted White" and "Tinted Colour 3" which to the naked eye is light yellow.
The carat is the weight of the Diamond and like most things in life, the bigger provides the greatest value. One carat is equal to 0.2 grams, so a 5 carat Diamond will weigh 1 gram. Larger diamonds are rarer, so in purchasing a Diamond you will find the price increases greatly as does the size.
Traditionally these are the 4 C's. If you are looking to sell your Diamond, any potential purchaser who will be looking to resale or use your diamond will find great confidence if your Diamond is certified by registered valuation companies who provide certificates to demonstrate the attributes.
Therefore the 5th "C" is the Certificate.