Duncan Trice's wedding ring is the big symbol in the Cass Mastern story. Like many of the symbols in All the King's Men, the ring is a powerful symbol of judgment.
First and foremost, the ring represents the betrayal of Duncan Trice. It also represents a deeper betrayal of human beings when they are put into slavery. Remember that Cass wore the ring around his neck while he was fighting to free slaves.
Furthermore, the ring symbolizes Phebe's humanization in Annabelle's eyes. Remember the intense moment when Annabelle tells Cass about finding the ring?
" – and there lay the ring on the palm of her hand – […] – and I knew it was his ring but all I thought was, it is gold and it is lying in a gold hand. For Phebe's hand was gold – I had never noticed how her hand is the color of pure gold. Then I looked up and she was still staring at me, and her eyes were gold, too, and bright and hard like gold and I knew that she knew." (4.88)
Annabelle realizes that Phebe, who she legally owns (like she legally owns the ring), is precious and beautiful, and her moral superior in every way. What she can't live with is the fact that Phebe knows this too. This is why Annabelle can't "bear [Phebe's] eyes upon" her (4.151). Annabelle has a deep shame with no visible outlet and this leads her to further degrade herself by selling Phebe "downriver," delivering her into sexual bondage and severing her from her husband.
For Cass, this episode becomes humanizing. He accepts the burden Annabelle hoists upon him, and sees a way to redeem himself. Cass believes he has found a way to alleviate his pain – from betraying his friend and being a plantation owner himself – and do right by everyone involved.
Though he can't locate Phebe, Cass does effect rapid change in the community by radical means. He frees the slaves on his plantation, and pays them to continue working. His plan doesn't work in the long run, but people like Cass are important because they try to break down barriers of inequity when the odds against them seem insurmountable.
In addition to being symbol of judgment, and an agent of both humanization and dehumanization, the ring is a symbol of hope. Cass wears this heavy-duty symbol against his chest until his death. The ring slowly makes its way to Jack Burden so that its story can be told.