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Diamond carat grading...

When diamonds are mined, large gems are discovered much less frequently than small ones, which make large diamonds much more valuable. In fact, diamond prices rise exponentially with carat weight. So, a 2 carat diamond of a given quality is always worth more than two 1 carat diamonds of the same quality. (Note that a 2 carat diamond does not appear to be twice the size of a 1-carat diamond when viewed from the top.)
Note: the diamonds illustrated does not show actual size.  Own pics to be inserted

While larger diamonds are highly prized, diamonds of equal size may vary widely in value and brilliance, depending on their qualities of clarity, cut, and colour.
The most important thing to remember when it comes to a diamonds carat weight is that it is not the only factor that determines a diamonds value. In other words, bigger does not necessarily mean better. All four Cs namely Cut, Colour, Clarity and Carat Weight must be balanced in order to arrive at a diamond that fits your budget. None of the 4Cs is mutually exclusive, nor is any one more important than the others.
To give you a sense of the relative sizes of different shape diamonds, you will find the chart below: This chart may not be to scale and it will depend on the resolution of your monitor. Do we want to include this

What does the unit carat represent?
Interestingly enough, it is the weight of one carob seed, a product of the locust tree. These seeds are nearly uniform in weight and therefore were used by early gem traders as a unit of weight by which to measure their diamonds.
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Carat weight is the physical weight of the diamond. A carat is a unit of weight measurement for diamonds and coloured gemstones. One carat is divided into 100 points, with each point equalling 1/100th (.01) of a carat (ct.).
1.00ct. = 0.2gr.