Hello...I just got my GIA certification and the plotting had the circles. After comparing the plot to the actual ring I saw that the circles were crystals. Hope this helps...
I think the large circular graphics are clouds. Diamond crystals within a stone are generally smaller.
If you look with a magifying glass at the grading report, you might notice that the cloud graphics, are actually dots to form what is a cloud, and a solid line circle would denote a crystal.
The large circles circumscribe the areas of the cloudy areas, but if you look at them at very hi mag, sometimes you can see that the cloud is composed of minute crystals...kind of like looking at astronomical photos of distant galaxies.
Many folks get spooked by these large circles, but the "bark" is usually worse than the "bite."...it always helps to see it in person.
The thing that fools me about pictures ONLINE, is that everything is so BIG!!!!! And when I go to see it in person, I go, "Okay, where is the rest of it!!!"
But, dont get me wrong, you have to MAGNIFY the stones to actually see the imperfections and the cut of the diamond.