The Dogon were a reptilian species with thirteen eyes.
The sixth eye was located on the back of the head and was nearly the size of a golf ball, although it looked more like a shifting blue marble. This eye allowed a person to look back clearly on their life and make better choices, especially if swallowed.
Oddly enough, Eugene Jones steadily gained more physical contact with the world after swallowing a sixth eye and then dying. He was able to influence the mind of Gwen Cooper and eventually became corporeal and seemed to float into an afterlife. (TV: Random Shoes)
When Eugene was 28, he decided to sell his Dogon eye on eBay. His friends Gary and Josh bid on it to cheer him up. However they accidentally won. They planned to buy the eye and resell it at a higher price. When they tried to take the eye by force, Eugene swallowed it and ran away. Though he was later killed, the eye brought him back as a ghost-like being, allowing him to help Gwen Cooper investigate his death. He remained halfway between life and death even when the eye was removed from him. Torchwood Three retrieved the eye. (TV: Random Shoes)