Pearls are often considered peers to the classic gemstones, but they’re obviously different. From their biological origin to their fragile makeup, there’s something that makes pearls quite an oddity in a world where most gems are valued for hardness and color saturation. There’s a bit more to the story.
So, let’s dive in with a brief history of pearls, and then we’ll discuss why they are, or aren’t considered a mineral!
Are Pearls Considered a Mineral?
Pearls are considered a gemstone due to their historical use, but cannot be properly classified as a mineral under the definition of the material due to a lack of crystalline structure and the presence of organic compounds.
A Brief History of Pearls
Pearls are a strange thing. Beautiful in their pale, almost ethereal, coloration they’ve been sought after for much of our history.
Everywhere that pearl oysters cropped up there were humans willing to brave the depths and take a look for them. While very few of the mollusks actually had pearls, those that were found made up a significant industry in many regions. In particular, Japan and the Persian Gulf both had extensive pearl industries.
Pearls were also found in freshwater mussels in the Americas, where the native population would often dive for them. The shells of the mussels were also used to produce small decorative objects in some cases.
Pearls are actually part of the way an oyster protects itself. Specifically, the genus of oysters that are most frequently exploited commercially is Pinctada. Not all oysters in this genus are capable of producing pearls on a commercial scale, however, several have failed to be exploited.
Pearl formation occurs when an outside body gets inside the shell of one of these mollusks. This may be a parasite, a grain of sand, or even bits of it’s own tissue after surviving an attack from a predatory crustacean like a crab. The embedded particle or cells then become covered in layers of calcium carbonate in two different forms.
The end result is the iridescent orb that we call a pearl. The process works the same way in the freshwater mussels that are also harvested for pearls in some places.
Pearls were a lucrative industry for those areas that contained them. Prior to modern cultivation methods, they were vanishingly rare, natural pearls might form in less than 1% of oysters in the wild. They also take years to grow.
Diving for them was a dangerous task as well. While some areas had oysters just offshore, at depths of 5-6 feet, most of the time divers would have to venture out further. Diving to depths of 30-150 feet was normal for those involved in procuring oysters. If you’ve ever done any free diving you’ll recognize the higher end of that spectrum as being… well, very deep for anyone without some sort of oxygen supply.
These days there’s less pearl diving that occurs in wild areas. You may have noticed that pearls really aren’t that expensive, and that has to do with modern cultivation methods. Instead of waiting for nature to do its thing, we now place small “seeds” of plastic or glass inside purposefully grown oysters.
Pearl farming isn’t all that new. The famous fish breeder Sergius Orata is credited by Pliny as having first figured it out and implementing it in the British Isles.
These mass farming techniques have created a different sort of market for pearls. While some varieties, particularly the black Tahitian pearls, are still costly on an individual basis the ubiquity of cultured pearls has made all of them rather cheap. No longer limited to royalty, anyone with a debit card and a couple of dollars can order long strings of the cheaper cultured pearls for themselves from such obscure sources as Amazon or eBay.
That said, they remain beautiful and are still used in fine jewelry to this day.
What is a Pearl Made Of?
Pearls are made up of the initial irritant, and then layers of different forms of calcium carbonate combined with a substance called conchiolin. The latter substance is a combination of proteins that bind together the layers of calcium carbonate within the pearl itself.
Generally, there are two different forms of calcium carbonate contained within a pearl.
The interior portion, just outside of the nucleus, is formed of columnar calcite or aragonite. This forms a small yellow ball around the nucleus. As it grows, the outer layers of the pearl convert to tabular aragonite, which is known as nacre or mother of pearl. Perfectly spherical natural pearls are quite rare, but they’re generally round to some extent.
Tabular aragonite comes in very thin layers. These layers scatter light as it reaches the surface, causing the pearlescent effect on the surface of the pearl. The thinner the layers, the better the sheen, since the light will pass through more of the layers and scatter more.
This is the same substance seen on the inside of mussels, abalone, oysters, and other shellfish.
The rockhound who is paying attention at this point will see that both calcite and aragonite can be contained within the pearl. Since these are both relatively common minerals, it may seem that pearls should be classified in the same way.
That doesn’t mean that a pearl is a mineral. Indeed, pearls aren’t considered a mineral by… anyone.
The Reason Pearls Aren’t a Mineral
There are two main reasons that a pearl is not considered a mineral.
The first, and probably most important, is the fact that pearls lack an orderly internal structure. It’s the same reason that opal is considered a mineraloid. In short, they aren’t crystallized so they don’t quite meet the definition of a mineral.
That may seem strange, considering the combination of layers of conchiolin and nacre in a seemingly ordered fashion, but the organic formation of a pearl precludes it from being a crystal. The forms of aragonite and calcite found in the Earth do have this internal arrangement, so they are considered minerals.
The second reason is the biogenic origin of pearls. They come from a living thing, in other words, while minerals are generally found in the ground. We don’t consider our bones or teeth minerals, for instance, even though they contain some minerals.
Conchiolin is the specific reason why pearls can’t be considered a mineral. This complex blend of proteins is essential to a pearl’s formation, but it’s a biological material to the core.