One of the stranger lots that Martin Nolan, a co-founder of Julien’s Auctions, ever put on the block during his more than two decades in the auction business was a thin slice of cake served at the July 1981 wedding of Diana, Princess of Wales, to Prince Charles. It sold for $2,831.

That was in 2008. Today, Mr. Nolan estimates that it could fetch tens of thousands of dollars.

“We’ve noticed over the years that Diana is highly collectible as an asset class,” Mr. Nolan said in a recent phone interview. “She’s considered a blue chip as an investment. Also, people feel they have a connection to Diana.”

The world had just 16 years to get acquainted with the princess, Mr. Nolan said, referring to the time between her official entrance into public life and her untimely death in 1997. The brevity of that period, he said, has only intensified public fascination. Many now seek out Diana memorabilia in an effort to preserve the connections they feel they had with her.

That demand has pushed auction houses to put an increasing number of her belongings up for sale. This Thursday, Julien’s is auctioning a large collection of her personal effects, alongside a selection of items from other royals including Queen Elizabeth II and Princess Margaret. On offer are Diana’s Lady Dior bag (which was renamed as a tribute to her), dresses and hats and a group of more intimate pieces such as personal letters, birthday cards and holiday cards. A portion of the proceeds from the sale will be donated to Muscular Dystrophy UK.

At high-profile auctions that follow the death of public figures — and continue years later, provided demand remains — items like clothing, furniture, jewelry and photographs are standard. But deeply personal objects have become increasingly common.