In some editions yesterday, the names of two guests at a benefit for the Friends of the National Arboretum were reported incorrectly. They are Cynthia Darlington and Harry Darlington IV.

As one guest put it, "When Ethel Garrett called and asked me to come, I didn't even ask what the cause was. I just said, 'I'll be there.' "

Garrett has supported so many Washington civic causes that when she put out the call for last night's Friends of the National Arboretum benefit, at the Madison Hotel, 230 checks were in the mail for the $250 tickets almost before the invitations were sent.

Unlike most parties in the capital, this cause was about the history of Washington, the city, and most guests were gilt-edged, card-carrying Washington cave dwellers, as Washington's old society is often called. But the entertainment was Out of Town, Out of This World and Outrageous: $50 million worth of jewelry from Harry Winston and $2 million worth of shining costumes by Hanae Mori.

The evening benefited a cause that Garrett, the honorary chairman, has championed for a quarter of a century. In 1962, the old East Front of the Capitol was lost in an expansion. Twenty-four columns topped with Corinthian capitals, designed by Capitol architect Benjamin Latrobe and originally cut from George Washington's quarry, were removed from the East Front Portico and dumped on the banks of the Anacostia River.

Garrett had the stones crated to preserve them and started a campaign to have them re-erected. Last night's profits will go to help pay for an acropolis, designed by landscape architect Russell Page, to be built with the columns on the grounds of the Arboretum.

"President Reagan promised me, we're on our way," said Garrett. In a shimmering pink and purple Hanae Mori, hand-delivered by the designer, she held court on a love seat during the cocktail party before the pheasant and Grand Marnier souffle' dinner.

Henry M. Cathey, director of the Arboretum, toasted the benefit chairman, Cecil Carusi, and Garrett. Afterwards, he said the design would cost about three-quarters of a million dollars. "We're going to erect it on Mount Garrett," he said. "At least that's what I call the spot, in recognition of all of Ethel Garrett's efforts." Even with the approval of Reagan and a push from Vice President Bush, he said, it will probably be at least two years before they will be in place.

Stones of another sort provided gleam and glitter to the evening. The 94.8-carat Star of the East diamond, which once rubbed against the Hope diamond on the bosom of Evalyn Walsh McLean, hung last night on a model in the Hanae Mori fashion show. The pear-shaped diamond was lent by Ronald Winston, head of Harry Winston jewelers.

The jewels, including some incredible emerald necklaces, one worth $5 million, complemented 100 costumes by the Japanese designer who shows in Paris and New York.

Ronald Winston winced every time a flashbulb went off. "I don't like to be photographed, ever."

Like all diamond merchants? "Like my father [Harry]. I don't want to be kidnaped. Or robbed."

Winston wore perhaps the tiniest diamonds of his collection, set in square fluted crystal studs rimmed with more diamonds, made in 1910. "My father's," he said.

Cynthia Darling, Garrett's granddaughter, said the evening came about because her brother, Harry Darling IV, who went to college in Japan, now works for Hanae Mori. Darling was wearing a Mori, as were with Annemarie Pope, Mary Ripley and numerous others.

Japanese Ambassador Yoshio Okawara and Mitsuko Okawara watched the show, proud of their compatriots, Hanae Mori, her husband, Ken Mori, and their son, Kei, who heads her American office. Zhang Zai, China's minister counselor here, said, "Maybe one day we will have clothes like these, but we will keep our own character."

Sotheby's was out in force. The Earl of Westmoreland said the firm's board had just learned it will be auctioning the painting collection of Florence Gould in April from her homes in the south of France and New York. Michael Ainslie, the former head of the National Trust for Historic Preservation and now chief of Sotheby's, was exuberant over the Gould news. "I'm working harder," he said, "and enjoying it more."