An NFT of the World Wide Web sells for $5.4 million.

In the past six months, auctioneers have sold a computer-generated illustration by Mike Winkelmann, the digital artist known as Beeple; digital Hot Wheels art made by Mattel; and even a New York Times column.

But the highly speculative market for NFTs has also cooled in recent months.

On May 9, the price of Ether, the cryptocurrency to which the values of NFTs are pegged, soared to $3,883, more than five times its price at the beginning of the year, according to coindesk.com. At that point, weekly completed sales of NFTs at dedicated marketplaces on the Ethereum blockchain (which do not include Christie’s and Sotheby’s auctions) reached a high of $176 million, said nonfungible.com, which charts the performance of the NFT market.

By May 20, weekly NFT sales had slumped to $19.2 million, a decline of 89 percent, and have flatlined at below $20 million, according to the database. The price of Ether has fallen, too, dipping below $1,800 in June.

This correction in NFT sales, or “stabilization phenomenon,” as the nonfungible.com blog preferred to call it, was due in part to the steep fall in the price of Ether as well as the age-old, ever-irrational cycle of boom and bust.

“Things have settled down,” said Anders Petterson, co-author of the “NFT Art Market Report,” published in May by ArtTactic, a London-based art market analysis company.

“The NFT market reached such high levels that people began to question where was the value,” Mr. Petterson said. “It got saturated. There was a massive supply of new artists, and we don’t have a qualitative benchmark. If you can’t explain value beyond the fact that people are buying it, it becomes difficult.”

But the top-end traditional auction houses, with their formidable global marketing machines, continue to set one-off benchmarks for NFTs. In June, when the specialist marketplace for “nifties” was still supposedly in the doldrums, Sotheby’s sold a rare “alien” CryptoPunk for a record $11.8 million, the second-highest price ever achieved for an individual NFT.