For any gemstone, the measuring unit of its weight is Carat. A carat is further divided in to 100 points. For example, a diamond measuring 50 points is ½ carat in weight or 0.50 ct. A diamond measuring 75 points in weight is ¾ carat or 0.75 ct in weight. The grading of a diamond begins right from this point.
The easiest way to determine the weight of a diamond is this: 1 carat is equal to 0.2 gram.
Again, there is wide range of possibilities that two diamonds of the same shape cut from the common rough piece can differ based upon the level of proportions.
The pricing of diamonds don't follow static logic like bullion market. For example, one carat diamond costs more (much more than double) than 2 times ½ a carat diamond as it’s rare to find a large piece of diamond.
Clarity is measured on a scale ranging from pure (flawless) to heavily included (I-3). The clarity of a diamond is graded by using 10X magnification under good lighting by an experienced grader.
The final clarity grade is usually determined by how easy the inclusions and blemishes are for the grader to see.
The popular tools used for clarity grading are the hand held loupe or microscope with 10 x magnification system. Hence, we can say that the imperfections not filtered under 10 x magnifications are not considered as imperfections.
Ideally, a diamond should be a transparent gem piece. But still, as some of the stones are found with Color, the process of grading through colors become more important. The diamonds are measured on a scale ranging from no Color at all (D) to deeply colored (Z).
After ‘Z’, the diamonds are found rich in colors which are popularly known as "fancy colors". The diamonds of popular Color are used as comparison stones for Color grading. A machine called the "Colorimeter" is being used in Color grading at some places but nothing can beat the trained human eye.
As we all know that the valuation of a diamond is not just finalized by its color grade. Hence, sometimes a visible tint in K and above can still be more beautiful and of better value if it carries better clarity and cut. Only diamonds graded N onwards have a tint that is visible through naked eyes.
The most popular color tints are combinations of pink, blue, brown, yellow, orange, green and red. Among these yellow is the most common shade while other rare shades are red, blue and green.
Most of the times forgotten or ignored "C" is the cut of a diamond which ensures brilliance and sparkle of the gemstone. Cut is used to grade a stone for its overall appearance. For a non technical person, very simply, if a diamond cannot catch your eye or attracts you in a flash time, it is not a good cut. A piece of diamond cut perfectly blinds you with its charm.