The ring is more of a doorway, with its rim as thick as an arm, a height of over a span, and a width of about a pace. It glitters in ever-changing colors.
This ter'angreal is what is used to test a woman to become a full Aes Sedai. A number of channelers must channel all Five Powers in a complex weave into it, while the candidate enters the rings into a mirror world and must successfully perform one hundred weaves at the right times to pass.
In the test for Moiraine the Oval was activated by seven Aes Sedai, one from each Ajah. In Nynaeve's test there were seven Sitters and the Amyrlin Seat. In both cases the Sister involved were strong in Saidar, but it is not stated if a minimum amount of power is required to participate in the activation.
Injury and death can occur within the rings, and a person can also simply not emerge from the ter'angreal.