The factors that determine the value of a diamond are the carat, colour, clarity and cut, referred to as "The Four C's".
Carat refers to the size of the diamond in terms of weight. 1 Carat (Ct) weighs 1/5th of a gram. 1 Carat can be divided into 100 points, just as $1 can be divided into 100 cents. Therefore, 1/2 carat equals 50 points; 1/4 carat weighs 25 points and so on. The more a diamond weighs, the rarer the diamond, the more you will pay.
Diamonds are over 95% comprised of carbon. Other minerals get trapped in the diamond when it is forming. Often these trace elements alter the colour of the diamond. When nitrogen is trapped in the diamond the diamond begins to turn yellow. The colour scale is from D-Z. As you go from D to Z with D being the whitest there is more nitrogen present, the diamond becomes more yellow, the diamond is less rare and therefore costs less money.
Most often, we see traces of the natural growth process in the finished diamond product. These characteristics are called blemishes or inclusions. Blemishes are characteristics such as scratches that are confined to the surface of a diamond. Inclusions (trapped minerals, breaks, growth marks), however, are either totally confined to the inside of a diamond, or they start inside a diamond and break through its surface.
The clarity scale ranges from Flawless to Imperfect.
The in-between grades assess how apparent the inclusions are when viewed through 10X magnification. The fewer inclusions, the rarer the diamond, the more you will pay for the diamond.
FL - Flawless: No blemishes or inclusions when examined by a skilled grader using 10X magnification.
IF - Internally Flawless: No inclusions - and only insignificant blemishes - when examined by a skilled grader through 10X magnification.
VVS1, VVS2 - Very, Very Slightly Included: Minute inclusions that are difficult for even a skilled grader to locate through 10X magnification.
SI1, SI2 - Slightly Included: Noticeable inclusions which are easy - or very easy - to see through 10X. In some instances, even, inclusions can be spotted with the unaided eye.
I1, I2, I3 - Included: Inclusions which are obvious to a trained grader through 10X magnification, can often be seen face up with the unaided eye and could potentially affect durability or beauty.