Every diamond is unique but the common factors which determine a diamond’s quality and price are described by the 4 C’s; CUT, CLARITY, COLOUR and CARAT WEIGHT. Understanding these variables will help you get a better sense of what you personally value in a diamond.
 

CUT – Not to be confused with shape (i.e. round, oval, princess), the cut of a diamond affects the brilliance, fire and scintillation; in other words, how much a diamond glitters. Ranging from Ideal to Fair, Cut is the only "C" under human control. A diamond which is "ideal cut" captures and releases the maximum play of light. A cut too deep or too shallow will allow light to leak out and not be reflected.

CLARITY – Clarity is the degree to which tiny marks of nature are present in the diamond.  The fewer marks the stone has, the more valuable it becomes.  Characteristics that occur on the surface are called “blemishes” and inside the stone are called “inclusions”.  Clarity is determined by the number, size, nature, and position of these inclusions, from flawless (FL) to heavily included (I3).  A “flawless” diamond has no blemishes or inclusions visible under 10-power magnification.  In terms of appearance, diamonds present an almost endless variety of clarity-characteristic combinations, so there can never be one single description that automatically describes a given grade, and becoming skilled at grading these characteristics takes time, experience, and practice.

COLOUR – Diamonds exist in every colour of the rainbow and intense colours make the diamond very valuable (called fancies). The value of most diamonds is based on the absence of colour. This system ranges from D (colourless) and continues with increasing presence of colour to Z (light yellow). Fancy colours are graded on a separate colour scale.  The more colourless the diamond, the greater its quality and price.  Stones are viewed in the face down position because brightness and fire are minimized in this orientation, making it easier to detect colour.  Each grade represents a range in colour rather than a specific point.  

CARAT WEIGHT - This is the standard measurement for diamond size. The most obvious variable, it is actually the least important in determining the brilliance and beauty of a diamond. This system began with the small, uniform carob seeds used by early gem traders as counterweights for their balance scales. Each carat (ct) weighs 200 milligrams and is subdivided into 100 “points” (pt). A diamond weighing 0.25 carats (0.25ct) is the same as a quarter carat (¼ct) and the same as 25 points (25pt). Don’t confuse carat with karat, which measures the purity of gold.