When Marjorie Merriweather Post traveled to London for the coronation of Queen Elizabeth II in 1953, she brought along some of her fabulous jewelry — and, as one of the richest women in the world, we’re talking VERY fabulous. At one of the pre-coronation parties at Buckingham Palace, Post wore emeralds — including a hard-to-miss 21-carat ring known as the Maximilian Emerald.

As she was leaving the party, she slipped into her car, looked down at her hand and saw that the emerald had somehow fallen out of the setting. “So they immediately called the palace,” recalled Post’s granddaughter Ellen Charles. “I suppose only at Buckingham Palace would they find the stone.”

The improbable lost-and-found story had a happy ending — not only did Post get the beautiful Colombian emerald back, but it’s on display at “Spectacular,” an exhibition of 50 pieces of jewelry owned by the late cereal heiress. The show, which opens Saturday and runs until the end of the year, was previewed Tuesday at a black-tie dinner on the lawn of Hillwood, her Washington estate and now a museum. Post bought the Maximilian Emerald (named for its first owner, Emperor Ferdinand Maximilian Joseph of Mexico) in 1928 and later donated it to the Smithsonian’s gem collection at the Museum of Natural History. Now it’s back home, along with a dazzling diamond necklace Napoleon gave his second wife, Marie Louise, when their son was born, and a pair of huge pear-shaped diamond earrings once owned by Marie Antoinette.

No word on the value of the collection — seriously, Hillwood officials aren’t allowed to say — but we’re guessing hundreds of millions.