A ring that belongs to the woman known as Dr. Charlotte Cross.
In Chemistry, Audrey Parker and Vince Teagues find the ring hidden in a glove compartment in the car of Charlotte Cross. When they confront Charlotte about this, Charlotte demands the ring back and asks Nathan to give it back to her. As Nathan gives the ring back, he feels her touch. Charlotte tells them that this ring belongs in her family including her daughter Mara.
In New World Order, Charlotte reveals that her ring is the key to opening Thinnys and going back to the other world. In Wild Card, Charlotte gives Dwight her ring before she dies at the hand of Croatoan.