The Four "C"s
Two diamonds of equal weight can be unequal in value.  Superior cut, whiter color, surface clarity and carat weight all contribute to a greater diamond value.  The following categories give you an idea of what to look for when choosing a diamond.
Color	A whiter diamond is considered to be colorless. The less color a diamond has the more valuable it is. Diamond color ranges from whiter, or colorless, to faint yellow to strong yellow. Diamonds color is determined by viewing a diamond from the side and comparing the color to predetermined color values of GIA certified master diamonds.  These GIA master diamonds hold alphabetical values ranging from D colorless through Z fancy yellow.
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Clarity	The clarity GIA clarity grade determines a diamond's purity, beauty and is a determinant of value. Clarity describes the presence or absence of inclusions on the surface of a diamond. The fewer inclusions a diamond has the more rare and valuable it is. Inclusions, or blemishes on the diamond's surface, interfere with the light passing through the diamond, therefore diamonds with a lot of inclusions are less brilliant because the light cannot pass through smoothly.
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Cut	Cut and cut precision determines the value and brilliance of a diamond. Accurately cut diamonds attract the highest price per carat. Light will enter into a diamond, reflect from its upper facets and return back out of the top in an even balance of white sparkle if a diamond is well-cut with accurate proportions.
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Carat	A carat (or karat) equals 1/5th of a Gram.  One carat is divided into 100 points.  For example a 50 points weighs 0.50 carats. As stated above, the size of a diamond is not the only factor in determining the value of a diamond. The weight of a diamond is expressed in carats.