Green diamonds are one of the most difficult diamonds to value and understand on the colored diamond market. The reason for this is the way in which green diamonds are formed. Caused by exposure to natural irradiation during the diamonds formation, the color of green diamonds can either be throughout the stone or just present on the surface.
What expert cutters are now doing is leaving "skin" or surface radiation staining on the stone to give the appearance that the stone is greener than it is. Because the GIA grades color on face up appearance and because the skin is natural there is little indication how the stone appears green.
Generally how one can tell is to view the diamond loose and see how much of the stone is in its rough form, the less the better. Green diamonds with the same GIA grades can be fractions of one another in value because of green "skin". This should not deter a collector from purchasing a skin stone however because a natural green diamond is extremely rare in any form.
Green diamonds are found predominately in regions of Africa and South America. The Dresden Green is the most famous green diamond. Weighing approximately 41 carats, it is often referred to as the cousin of the Hope Diamond for its historical importance.