If you break an engagement, give the ring back. Once married it's yours.

He turned into a frog.
Prince Charming No. 2: Frog.
Prince Charming No. 3: Frog.
After Roth’s third divorce, she sold the ring Frog No. 3 had bought her and purchased three thumb-size gold and diamond frog pins. She wears them crawling up her blouse in a column toward her neck.
“My mother always told me I would have to kiss a lot of frogs,” she said. “Instead, I married them. So this was an inexpensive way to a new beginning.”
The story of post marital jewellery has many strands. 
There are the emotional tales of rebirth and re purposing, such as Roth’s frogs. There are the stories of the strictly financial, like when a piece that was once a symbol of everlasting love morphs into a strictly salable commodity that helps to pay the mortgage, a child’s college tuition or a charitable donation.
Upon her divorce from Donald Trump in 1999, Marla Maples sold her 7.45-carat Harry Winston diamond for $110 000 and reportedly donated the cash to charity. It was a move Trump called “pretty tacky.”
And then there are the tales of bitter court disputes, once the battle of the assets commences. Disputed jewellery is sometimes lied about, hidden, stolen and, in rare instances, brazenly worn in public.
“Horses, wine and jewellery — they always seem to disappear,” said Nancy Chemtob, a matrimonial lawyer, herself divorced. 
“One client has a wine collection and the divorcing spouse has a party and the wine is all gone. The horse always dies. The jewellery gets sold, even though I wonder if he or she actually sold it. We never really know.”
Divorce filings typically surge at the turn of the new year.
“I know a lot of people feel it’s the stress of the Christmas holiday,” said Madeline Marzano-Lesnevich, a lawyer. “But I personally think that people say to themselves, ‘It’s a new year, I want to start a new life for myself.'”
Limor Shaya-Rosenberg said she lied about the jewellery (yes she did) after her 18-year marriage snapped like a pearl necklace.
“I told my ex-husband I sold the gold Rolex he bought me in the Bahamas to pay for the divorce lawyer, but he knew I had it. He saw it on me,” Shaya-Rosenberg said. “I’m wearing it right now.”
“He said he bought the Rolex for himself, so it became a battle at the beginning,” Shaya-Rosenberg added. She said initially she held out hope that their problems might work themselves out, then realised that selling the jewellery meant the marriage was, at least for her, finally over.
A need for money prompted Shaya-Rosenberg to sell a Tiffany charm bracelet, diamond earrings, a diamond horseshoe pendant and more.
She even sold her diamond engagement ring to send her three boys to camp.
She has entered the realm of ex-spouses who seek to make a profit off their spurned jewellery, in an embarrassment-of-riches sort of way. Now that she has a son in college, she’s looking to sell a diamond Cartier watch
In a broken engagement, a ring’s return is often requested. “If the marriage doesn’t happen — let’s assume it’s a man and a woman — she has to return the ring,” Chemtob said. “If they get married, it’s hers.”
-The New York Times