At a 2018 Sotheby’s auction, an exquisite pearl and diamond pendant worn by Marie Antoinette sold for $32 million. Naturally, the provenance of the piece influenced the final price, but it’s worth noting that pearls were rarer than most other gems in the doomed French queen’s time, including diamonds. Without the history, though, would the pearl jewelry have been worth as much on its own?

“A pearl is probably the most complex gem to assess,” says gemologist Tom Moses of the Gemological Institute of America in the video series So Expensive from Business Insider.

Moses noted that one reason the Marie Antoinette pearl sold as well as it did was because it was a natural pearl, which ranks as the highest of the three pearl classifications: natural, cultured and imitation. All are beautiful, but some are more precious than others.

Natural Pearls

Imagine you are a mollusk – an oyster, mussel or any type of shelled bivalve – and an irritant enters your shell. You don’t have teeth, claws or other built-in weaponry. Your only defense is to cover the invader, layer by layer, with a calcium carbonate (conchiolin and aragonite) substance you produce, called nacre (NAY-ker), or mother-of-pearl. You keep excreting nacre until the unwanted visitor is completely surrounded and no longer a threat. This irritant also serves as a bead nucleus, eventually growing large enough to become a pearl. The word “pearl” is derived from the Latin word perna, meaning leg – the shape of a bivalve known to yield pearls.

Until relatively recently, the only way to obtain natural pearls was to recruit a deep-water diver to retrieve them. The work was dangerous and deadly. Some divers attached weights to their feet to speed their plunge to the seabed, where they would gather all the oysters they could grab while holding their breath. They would return to the surface, deposit what they had caught, and repeat the process.

The arrival of the metal diving suit made things slightly easier, but it, too, could prove hazardous if its air hose became disconnected or there was some other mishap that required the diver to be pulled up to the surface. In any event, fishing for natural pearls is life-threatening work with no guarantee of a reward. For every ton of oysters harvested, only a handful of pearls is found.

This explains why natural pearls are so expensive and why only about 1% of pearls in circulation qualify as natural. In fact, most natural pearls are vintage and almost never appear outside of auction settings. Designers of contemporary pearl jewelry rely on cultured pearls instead.

Cultured Pearls

By the late 19th century, overharvesting of oysters for pearls and food, combined with pollution that resulted from the Industrial Revolution, led to the decline of natural pearls.

In 1893, Japanese entrepreneur Kokichi Mikimoto filed a patent on a method of creating cultured pearls. By the 1920s, after much trial-and-error and after negotiating permission to incorporate other patented methods in combination, Mikimoto mastered the production of spherical, naturally occurring cultured pearls – a feat that many had thought was biologically impossible.

The process involves carefully inserting round pieces of organic oyster material or shell, called saibo, into another pearl-bearing oyster or mollusk mantel as a bead nucleus. Thus prepared, the mollusk or oyster is reintroduced into either freshwater or saltwater until it generates enough nacre for the desired size of pearl, which can take between six months to two years. Mikimoto’s approach is still the basis for cultured pearl production, more than 100 years later.

Imitation Pearls

Costume jewelry pearls are usually manufactured from glass, ceramic, shell or even plastic bases that are painted or covered with mother-of-pearl to simulate the iridescent luster of a pearl. They draw little to no interest at auction.

How to tell the Difference

Just by looking at a gleaming, soft, smooth, iridescent pearl from afar, it can be difficult to know precisely what type of pearl it is. Is it a natural, cultured or an imitation pearl?

It is perfectly acceptable to drag it lightly across the surface of your teeth (not the edges, or else you might scratch the pearl) to see if it feels a bit rough and uneven. If it does, it’s either a natural or a cultured pearl. An imitation pearl will have a smooth surface.

Gemologists use X-rays to determine a pearl’s origin. Natural pearls will show layers of growth, similar to the interior of an onion, with the bead nucleus at the center. Cultured pearls, in contrast, have only one thin layer of growth at the top. Freshwater pearls show no evidence of a bead nucleus, because it will have been completely dissolved.

Imitation pearls lack the heft of both the natural and cultured pearl and are easily scratched. Also, its manufactured covering can peel under the stress of constant use.

What to Look For

Pearls are classified by size, shape, color, luster and complexion and are measured in millimeters, usually from 5 mm (1/16in) to as much as 21 mm (13/16in).

The size of the mollusk will determine the eventual size of the pearl, but the most desirable shape is round, as round as possible. Only about one out of every 15,000 oysters produces a pearl of ideal roundness, according to gemologists.

Freshwater pearls come in a variety of colors, with the best classified as looking more pinkish than greenish, according to pearl grading charts. Luster is key to grading a pearl – it must reflect light clearly. Finally, the complexion of the pearl should be free of dents, ridges, edges or any marks to earn gem-quality status.

Cultured pearls are classified according to where they were grown. Freshwater pearls are cultivated from margaritiferidae mollusks, most of which are from China, while saltwater pearls come from pinctada oysters primarily farmed in Japan, Australia and the South Sea islands. Neither of these mollusks or oysters are the types we eat, so there’s no risk of accidentally biting into a pearl at dinner.

The smaller Akoya saltwater pearls, which come from Japan, have the highest luster of all cultured pearls and boast a desirable creamy white color. Japanese saltwater pearls classified as Hanadama are certified as having a higher gem quality. South Sea and Golden South Sea pearls are regarded as the rarest of the saltwater pearls. They feature a distinctive creamy silver luster and are cultivated in much larger sizes.

Freshwater pearls began to appear in the 1990s. They deliver more vibrant colors, distinctive shapes, and different sizes than their saltwater counterparts. They are much more plentiful as well. The variety known as the Baroque pearl, for example, has an irregular but pleasing shape, while the keshi or mabe (MAH-bay) pearl has a more flattened appearance, with a so-called “blister” in its center that readily accommodates a decorative gem.

The Tahitian pearl is cultured from the black-tip freshwater oyster producing the scarce black pearl. Dubbed the “Pearls of Queens,” Tahitian pearls usually assume irregular, baroque shapes, with perhaps one out of 10 presenting as round. Their colors range from black to gray, brown, green and even purple.

Whether natural or cultured, pearls have graced kings, queens and nobles for thousands of years. Pearls have gained even more appeal in modern times for being an environmentally friendly gemstone, the product of a renewable source cultivated in protected, unpolluted waters. They require little interference by man-made methods and provide ample luster without so much as a polish.

Whether acquired for investment, gala nights out or everyday sophistication, pearls possess a serene, magical essence that was reportedly described as “tears of joy” by Aphrodite, the Greek goddess of love. No matter how pearls are worn, they will look and feel timeless for generations to come. That is Nature’s way.