I dont understand buying anything over an H color or VS clarity. unless you put the loose stone under a microscope it's going to look just like a D color FL clarity. Plus it will cost less for a larger stone!
Not all diamonds are of the same quality! GIA created the (Color, Clarity, Cut and Carat Weight) as a universal method for establishing the quality of any diamond, anywhere in the world. These are are scales for color, clarity and cut.