The price of a diamond can vary based on its size and quality, both of which can drive its price up. Carat determines the total weight of a diamond; cut influences the way the diamond sparkles; color refers to how clear a diamond might be (the less color, the higher the grade and as a result, the higher the cost); and clarity, or the imperfections that you might be able to see, either with the naked eye or through a microscope.
Other factors that might play into the cost of a diamond is the shape in which it is cut (which could also affect its quality), and whether the diamond has a certificate says it has been independently valued, assessed, and graded by experts certified by the Gemological Institute of America (GIA) (via Blue Nile).
So as far as an experienced jeweler is concerned, a 10-carat colorless diamond that's cloudy may not be worth that much more than a smaller stone with perfect clarity and brilliance.
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