CUT - Cut actually means two things, the shape of the diamond (round, marquise, pear, oval, heart, emerald, princess, radiant, etc.) and the proportions of the stone.  When a stone is cut to good proportions, light is reflected from facet to facet, then dispersed through the top.  The better the cut, the greater the sparkle, brilliance and fire of the stone. 

COLOR - Most diamonds look colorless.  But there are many subtle shade differences that range from colorless to yellow/brown.  Diamonds are graded on a color scale that ranges from D (colorless) to Z (yellow/brown).  Diamonds with no hint of color at all are extremely rare and are therefore, most valuable.

CLARITY- Almost all diamonds contain a combination of internal and external characteristics called inclusions and blemishes.  Normally, they are too small to be visible.  The number, type, location, visibility and color of the inclusions determine a diamond's clarity and may influence its value. 

CARAT WEIGHT - Like all precious stones, the weight or size of a diamond is measured in carats.  Carat weight is the most obvious factor in determining a diamond's value, depending on the quality of its cut, clarity and color.