The unlikely ingredient Swampscott cop Michael Serino nearly swallowed with his seafood soup could soon cough up a big bonanza for the married father of three.

A rare, 6.22-carat purple pearl that Serino fished from his mouth during a birthday dinner at a Portuguese restaurant will be auctioned March 15 at Kaminski Auctions in Beverly.

Online bidding, which kicks off Feb. 28, starts at $5,000 — but the prized pearl’s worth is estimated at between $10,000 and $15,000, said Harry Morgan, vice president of appraisal services for Kaminski Auctions.

Morgan said the sky’s the limit for the purple pearl. Last year, a man who brought in his grandmother’s 1820 decanters saw the bidding skyrocket from $2,500 to an eventual selling price of $39,500.

“So few of these come up for auction,” Morgan said of the natural lavender pearl. “It is just a stunning color.”

This fish tale began about six years ago when Serino, 46, and his family went to a now-shuttered Peabody restaurant to celebrate his birthday. Dad ordered the seafood stew.

“At first, I thought it was a rock,” he recalled yesterday.

He pulled the orb out of his mouth, took it home, put it in his daughter Julia’s jewelry box and completely forgot about it until recently.

“We didn’t know the value of it at that time,” Serino said.

Then in December, the couple saw a TV news story about a Virginia Beach woman who found a purple pearl in a littleneck clam — and her husband had estimated its worth at $3,000.

“So now I start doing my research,” Serino said, “and scramble around the house looking for it.”

Luckily for him, it was still in his daughter’s jewelry box.

Serino shipped the pearl to the Gemological Institute of America to authenticate it. According to GIA’s report, the natural pearl comes from a northern quahog clam and measures 11.43 by 8.36 millimeters.

“They are pretty special,” said Dona Dirlam, director of GIA’s gemological library and information center. “All natural pearls nowadays are quite special.”

Gina Latendresse, president of American Pearl Co. in Nashville, said she’s met many divers who have been collecting shells their entire lives and have never found a pearl of any kind, let alone a purple one. She estimated that there’s a 1 in 100,000 chance of finding a natural pearl the size and color of Serino’s gem.

Serino, father of Lauren, 22, Julia, 14, and Isabelle, 2, said he’d like a Corvette but his wife and daughters want a new kitchen.

He figures the kitchen is in the cards: “I’m going to get outnumbered by the girls.”