Carnelian agate from Botswana with several eyes. Image from http://rocktumbler.com/blog/eye-agate/

Lake Superior agate with one big eye and multiple smaller eyes. Image from http://rocktumbler.com/blog/eye-agate/

Close-up of two yellow eyes merging together. Photo from http://rocktumbler.com/blog/eye-agate/

Eye agate from Lake Superior, photo from http://agatenodule.com/rg-lak006ab.htm

An eye agate is an agate that is marked with one or more concentric circles, which look like eyes. The circles are usually a complementary color to the rest of the agate. The eyes are actually little spheres, possibly caused by silicification around a seed crystal of some other material located at the edge of the agate while the rest of the agate is forming. They just look circular because you see their cross-section. Sometimes two eyes will merge, creating a figure-8 inside the stone, as seen in the close-up photo.  Eye agates are pretty rare, but they seem to occur more often in agates from Lake Superior and carnelian agates from Botswana.