It’s a gem with a living past. Formed through millions of years of compression and mineralization, the ammonite has an opalescent quality and shimmers as prisms of light that are deflected from its sinuous outer shell. More than 70 million years ago, this clam was home to a creature now extinct, a cephalopod, which belonged to the squid family.

The vortex-shaped fossil found in the Canadian province of Alberta, which is more than 14 inches tall by 16 inches wide, is being offered for sale by the David Aaron gallery at this year’s Masterpiece Online Show.

It is not the only prehistoric natural object that is offered for sale. ArtAncient, a gallery that deals with antiques and increasingly with elements of natural history, offers a “gogotte”, a sandstone concretion that was formed around 30 million years ago near Fontainebleau, France, as water superheated by tiny grains of sand flowed and they merged into an intricate formation resembling a sculpture.

The market for natural history, and especially fossils, has exploded in recent years, even among collectors who normally buy art and antiques. “This is an area of ​​growth because I think people are increasingly interested in diversifying their collections and surrounding themselves with beautiful and historically significant objects that are imbued with meaning,” said Salomon Aaron, director of David Aaron. “Natural history is really a very, very fascinating area of ​​the art market right now.”

Dinosaur skeletons are the hottest commodity on the growing market. The latest iteration of the fossil frenzy may have started with Sue, a complete Tyrannosaurus Rex skeleton that sold for $ 8.36 million in 1997.

Then there was Stan, another complete T-Rex that was auctioned off at Christie’s last year for a record-breaking $ 31.8 million. James Hyslop, director of science and natural history at Christie’s, said the market has grown every year since 2007.

But it’s not brand new. Mr. Hyslop said this type of periodic enthusiasm had its roots in the 19th century. “Certainly the best fossils were on par with the most expensive paintings in the world throughout the Victorian era,” he said.

“The American fossil market is skyrocketing, especially when it comes to dinosaurs,” said Tom Lindgren, a natural history specialist at Bonhams, who held a natural history auction last month that sold a Tyrannosaurus tooth for $ 11,475 and up double butterfly in amber that cost nearly $ 38,000.

“When Christie sold the T-Rex, people got excited and wondered when the next Stan would be out?” he said. “It started a real dinosaur rush, like the late 19th century gold rush in California and the west. Now we are experiencing a dinosaur onslaught in this part of the United States. “

The American West is rich in fossils, especially in Wyoming, Montana, Utah, and Colorado.

And in the United States, it is legal to sell and export fossils found on private properties. Many other countries strictly regulate the sale of fossils or prohibit exports altogether, whereupon traders have to be careful when determining their origin. (For the past several decades, fossils have been illegally exported from Mongolia and China and stolen from public land in the United States).

“As a gallery, provenance is very important to us. Whatever we buy, we take extraordinary steps to be familiar with the provenance, ”said Aaron.

But the legal sale of fossils has also created dismay and alarm in the scientific community as more and more fossils end up in private hands at ever higher prices.

“The commercial market is not good for our science,” said Jim Kirkland, Utah’s state paleontologist. He has been trying for decades to track down a fossil fish skull that is vital to his research, but it’s nowhere to be found after disappearing into a maze of private hands, he said.

“These are world-class specimens, a lot of them come from Montana and Wyoming, from private ranch lands, and that’s perfectly legal,” Kirkland said. With fossils being sold for so much, more and more ranchers are leasing land to private excavators for additional income.

“Stan was sold for over $ 30 million,” he said. “When ranchers hear numbers like this, scientists can no longer work on ranches.”

Many dealers turn to museums first, Kirkland said, trying to avoid selling items of scientific value. “Anything that is scientifically important, we try to avoid auctions,” said Lindgren.

But the lines are blurry; Parts of what Mr Kirkland says he believes is a fossil of an as-yet-unidentified species of dinosaur are scattered across the United States, separated before the research could be done. “The advertisers didn’t know it was new and if they had they wouldn’t have sold it, but it’s piecemeal,” he said. “Sometimes it’s tragic. I just wish museums had the money. “

These issues may be less of a problem when it comes to specimens such as ammonites and “gods”, both of which have been extensively studied. But they remain rare and high-quality specimens are difficult to find which is part of the raffle for private collectors.

“In the late 1980s and early 1990s, many ‘gods’ were found at Fontainebleau, a site that is now closed to excavation,” so it is difficult to get fresh material from this site, “said Costas Paraskevaides, founder of ArtAncient . “Pieces of this size and quality are really very rare.”

The sculptural qualities of the great “god god” (25 x 44 inches) and the brilliant colors of the ammonite offered by Masterpiece make them interesting decorative objects, even for a collector who is perhaps less well versed in natural history and science. The “gogotte” costs 150,000 pounds (approx. 205,000 US dollars) and the ammonite 38,000 pounds.

A dinosaur skeleton can coexist with contemporary paintings and an Etruscan vase – making it a bold statement piece as many collectors seek.

Mr Lindgren said he recently worked with a private collector who had bought a 17,000 square foot home in Florida and wanted a woolly mammoth skeleton.

“They wanted a skeleton with a glass in front of the sea,” he said. “It’s an interesting part of the story, part of the theme of the house.”

But the appeal of owning a fossil differs from that of an art object, in part because of the wonder of its age. “It really puts things into perspective, the history of our planet, the nature of evolution, when you’re dealing with things that are 50 million years old,” said Aaron. “When you celebrate your birthday, you think about it.”