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The cut most directly effects the diamonds brilliance and appearance. A good cut will give a diamond its brilliance (and up to 50% of its value!). This brilliance is the reflective quality that adds the "fire" to the stone.
This fiery reflection is the direct result of the finished stones proportion and is seen when the light enters the diamond, reflects off the pavillion and sparkles up through the table.
In a poorly proportioned stone, the light does not reflect back upward and this brilliance is diminished or lost.
Note: Cut is not the same as the shape!
There are a wide variety of shapes that diamonds can be cut into. Take a look at the following shapes!
Picture source from www.diamond.com
The clarity of a diamond tells us the amount of inclusions (flaws) it contains. The impact on the diamond's quality is influenced by the size, number, location, and type of inclusion. Diamonds with very few inclusions are the most valuable as they are the rarest.
Note: Clarity grades F thru SI have an impact on a diamond's value only, not its appearance!
Diamonds come in every conceivable color; however, the color grade of a diamond usually refers to white (colorless) diamonds. When it comes to color in diamonds, less is definitely more. Colorless diamonds are the rarest and accordingly, they are priced the highest. However, it is important to note that while colorless stones allow more light through than colored diamonds (thus having more "fire"), fancy colored diamonds (red, blue, pink, etc.) can sometimes be more valuable.
The carat weight of a diamond is its size. The size difference between stones is easily discernable to even the untrained eye. Just remember, some stones are cut to poor proportions to either appear larger (too shallow) or to add carat weight (too deep).
1 carat = 200 milligrams
Read more at the following links.
Diamond Buying Guide