A fancy yellow diamond ring called the Marjorie Merriweather Post diamond was auctioned here today at Christie's for $540,000. The diamond, purchased by an anonymous London buyer, was the last of 37 lots of jewelry from the estate of Washington philanthropist Marjorie Merriweather Post, who died in 1973. The lots were sold with other pieces in an auction of "Magnificent Jewels."

The Post jewelry was put up for sale by the Marjorie Merriweather Post Foundation of D.C. to raise $1 million needed to build an addition to Hillwood, a house museum near Rock Creek Park that Post left to the public.

"We're going to use the money to build a replica of Topridge, Mrs. Post's Adirondack camp, to house her collection of Indian art," said a Hillwood spokesman.

The sale raised more than $1 million for Hillwood and construction of the addition is slated to begin in the fall. Altogether, the Post jewelry brought $1,282,550, although two unspecified lots were not owned by Hillwood. The most important lot of Post jewelry was the Post diamond, rated D, at the top of the D-to-Z scale used in rating diamonds.

A diamond tiara, which Post wore to many charity balls, sold for $130,000, exceeding the $70,000 Christie's expected.

More than 500 people crowded the elegant main salesroom at Christie's for over five hours, with diamond dealers clamoring and joking and well-dressed lookers ogling the jewelry shown on a large slide projector at the front of the room.

" This sale brought out more people than usual. The Post name was a big drawing card," noted Francois Curiel, head of Christie's jewelry department, who several times during the sale had to reprimand the enthusiastic crowd. She termed the sale prices "much higher than expected."

Other jewelry worn by Mrs. Post during her life as an international socialite can be viewed in the Mineral and Gem Room at the Smithsonian and at Hillwood, which also has her collection of decorative art, including a renowned collection of Faberge' boxes.

Also sold yesterday at Christie's, in a different lot, was a diamond ring said to have been in a bag around the neck of Emperor Maximilian of Mexico when he was executed in 1867. The ring, purchased by Laurence Graff of London, brought $660,000.

The sale figures do not include the 10 percent buyers premium charged by Christie's.