Go to any online diamond company that keeps an online inventory and compare prices for diamonds of the same colour, clarity, cut and carat. There will be a difference between two stones that on paper are exactly the same, but a company would not sell identical diamonds for different prices so what makes one diamond more or less expensive?
Quite simply it is the off-certificate factors that affect the price so greatly. Off-certificate means it is to do with the inherent qualities of the rough . This includes such factors as whether the diamond has a specific tinge that is not yellow (colour only measures yellow tinge), whether the stone is cloudy or milky (this again is seperate from on certificate data such as flourescence), and the luster or reflective index of the diamond. All of these factors drastically affect the price of the diamond but, unfortunately there is no way of you knowing what the specific off-certificate factors are of your diamond.
This means you could believe that you have a great deal on a diamond but in reality you have brought a diamond with good certificate grades but poor off-certificate performance. In other words, if it seems too good be true, it most probably is.
You can try and find out the origin of your diamond as Canadian and Russian diamonds are almost always formed in more favourable conditions that African diamonds. It is also fairly common for African diamonds to have a brownish or green tinge, when they would be shown as colourless on their certificate. Furthermore, Russian and Canadian stones generally experience less cloud or milkyness, and generally have a better luster.
These factors are also important when combined with your diamond certification, as a diamond with higher natural luster will have greater brilliance and scintillation as it’s ability to refelct light will be increased by a excellent cut, and obviously the better luster combined with the better cut leads to better scintillation and brilliance.