The 4 Cs of Diamonds:
It is a combination of the below 4 factors – Color, Clarity, Cut and Carat - which determines the value of a Diamond. It is not possible to value a stone based on any of these factors in isolation.
Carat for Diamonds (and indeed all other gemstones) is a measure of the weight of the stone – unlike gold, where the term pertains to its purity. The term carat is derived from carob, owing to the uniform size of the seeds from the Carob tree, which were used on precision scales as units of weight for small quantities of precious gemstones. The weight of an average carob seed is 200 milligrams. The weight of one carat is precisely 200 milligrams, or 0.2 grams. Smaller Diamonds, generally those under 1 carat, are also referred to in points, whereby 100 points make up 1 carat. Therefore a half carat stone can also be described as 0.50 points, and a 3/4 carat stone has 0.75 points.
The price of a Diamond does not increase on a linear scale with increases in the carat weight of a diamond. There are various weights above which there is a steep increase in value.
As a carat is a unit of weight and not size, two x 1 carat stones may appear to be different sizes.
The clarity of a Diamond is dependent upon the included particles, cracks etc， within the stone. Most Diamonds contain minute imperfections that occur when they are formed by nature. When the presence of these imperfections does not materially interfere with the passage of light through the stone, they do not affect its beauty and therefore have little effect on its value. For this reason, it is not only the size and quantity of any marks within a stone but also the positioning of any imperfections within the stone that is important.
The below diagram is an example of how the stones would be graded depending on their clarity.
Color in this context does not refer to a Diamond of definite color (these are treated separately and known as fancy colored Diamonds), but refers to Diamonds which are white but with a faint tinge of yellow or brown. The hint of color is often so slight that only trained eyes are able to detect it, and it definitely does not detract from the beauty of the stone.
The cut of a Diamond, often called the make, depends upon the proper proportions of the cut stone and the accurate alignment of the facet edges. In order to reflect all of the light entering the stone from the front, a well cut Diamond should have proportions as near as possible to a preset ideal.
There are a number of different shapes of cut. Brilliant, Princess, Navette, Asscher, Emerald and Pear are some examples.