The value of a rough diamond is calculated using four criteria:
By Carat Weight (Size). Originally the carat was the weight of a carob seed which ancient lapidaries used to balance their scales, as the seeds in a carob pod are very uniform in weight. This weight was standardized to the metric system as 1 carat = 200 milligrams (one-fifth of a gram). A carat is further divided into points, with 100 points to a carat. In general terms, rough and polished diamonds are weighed to two decimal places.
By Cut (Shape). A rough diamond can be classified into several main shapes. Stones (octahedron, dodecahedron), cleavage (broken stones), maccles, and cubes. Each of these main categories has a multitude of sub-categories and variations.
By Clarity (Quality). Quality is determined by how clean the diamond is inside. Very few diamonds can be described as “flawless”; most have inclusions which may be caused by carbon spots (piques), mineral inclusions (olivine), bubbles or cracks (gletz). The positioning of the inclusions within the diamond influences its value, as some inclusions may be polished out during the manufacturing process. Some diamonds may be so heavily included that it is almost impossible to see into them (Boart).
By Colour. Generally, the closer a diamond is to being absolutely colourless (white), the more valuable it will be. As the diamonds become more visibly yellow or brown, their value decreases. However, diamonds saturated with colour can be more valuable and are known as “fancies.” Diamonds can be almost any colour of the spectrum, red, orange, yellow, green, blue, pink. Impurities and different degrees of heat in which diamonds are formed cause various colours in diamonds.