1. Are Pearl Colors Made by Paint, or by Light Reflecting Like in a Soap Bubble?
You can see rainbow colors on a soap bubble because light reflects off both the front and back of the thin film, and the waves overlap. Pearl light is explained using the same idea. In a 2021 announcement, Ehime University stated that the color seen changes based on the thickness of the aragonite crystal layers on the pearl’s surface, which are a few hundred nanometers thick [1]. One nanometer is one-millionth of a millimeter. If a layer is 500 nanometers thick, it is about 1/140th the width of a human hair (calculated using a hair width of about 0.07 mm) [1]. Such thin layers determine the color.
Pearls can shine differently depending on the viewing angle. The way the color appears involves layers near the surface and the overlapping of light. These colors are called "structural color." They are not the color of the material itself but are determined by the thickness and arrangement of the layers. So, what are these layers made of, and how many thin plates are stacked together?
2. How Do Thin Plates of Aragonite Stack Up in Nacre, the Shiny Part of a Pearl?
The part of the pearl that shines is called "nacre" (or mother-of-pearl). Nacre is made of layered plates of the mineral aragonite and a small amount of organic matter like proteins [3]. A paper from a research lab states that by volume, about 95% is aragonite and about 5% is protein-rich organic material [4]. This structure is often compared to "bricks and mortar," where the plates are bricks and the organic matter is the mortar. However, the actual plates are much thinner than wall bricks.
The thickness of the plates is said to be a few hundred nanometers, which is close to the wavelength of visible light [1]. An encyclopedia lists a value of about 0.5 micrometers, but this varies depending on the type of shell and location [5]. If you divide 1 mm by a plate thickness of 0.5 micrometers, you get about 2,000 plates (this number decreases slightly if you include the organic layers). About 140 plates fit in the width of a human hair. It is the overlap of these arranged plates that turns light into color. So, how does the oyster stack these fine plates one by one?
3. How Do Oyster Cells Keep Adding Nacre Layers Around a Pearl's Core?
If you look at the cross-section of a cultured pearl, you see nacre layers forming concentric circles around the pearl’s core. The "pearl sac" creates this nacre. It is a sheet-like layer formed when mantle tissue cells wrap around the core. Organic substrate (protein sheets) and calcium carbonate plate-like crystals are secreted from the pearl sac onto the core’s surface and form alternating layers [2].
In the farming process, humans only insert a spherical core and a piece of mantle tissue into the oyster. After that, the pearl sac spends a long time stacking layers. Ehime University press materials state that pearl farming takes more than one year [1]. The thickness of the stacked layers is called "maki" (wrap) and is a key factor in the quality of Akoya pearl oyster pearls. A report from the Mie Prefectural Fishery Institute notes that the thicker the wrap, the higher the value, and it relates to luster and interference colors [2]. This system works because humans discovered how to make the oyster stack layers. How was this method found?
4. How Did Kokichi Mikimoto Find a Way to Make Oysters Produce Pearls?
Originally, pearls were found by chance inside natural shells. Kokichi Mikimoto searched for a way to make oysters produce pearls. According to stories, after much struggle, he succeeded in farming semi-spherical pearls in 1893 and took out a patent in 1896. There were failures along the way, such as red tides killing all the oysters [6]. Perfectly spherical pearls were said to be successfully produced in 1905, with a patent obtained in 1908 (some sources mark 1908 as the completion date) [6][7].
This research was not done by Mikimoto alone. A nippon.com article introduces that it progressed in competition with others like Tokuichi Mise and Fujikichi Nishikawa [7]. The pearl farming technology developed by Mikimoto is still widely used today, about 120 years later [7]. Ehime University press materials describe pearls as "the only gemstone that humans can create" [1]. While this is the university’s expression and definitions of gemstones vary, the fact that humans help the oyster make them is a major feature of pearls. The growing technology exists. So, how much do we know about the principle of their shine now?
5. How Much Do Scientists Understand About Why Pearls Shine?
The study of pearl color is very old. A book written by Brewster in 1833 also describes interference colors. However, Ehime University states that a complete understanding of the principle has not yet been reached [1]. Until now, the idea of "Bragg reflection," where light reflecting off thin layers reinforces itself, has been used. The 2021 Ehime University study added to this by considering that the pearl’s core, which strongly scatters light, acts as a pseudo-light source. In this model, interference colors are determined by the overlap of light reflected from the pearl’s surface and light that passes through from the inside [1].
Because farming takes more than one year, the university explains that it would be useful for research and development if simulations could predict what appearance results from what kind of layers. This model was presented as an optical model for predicting appearance, not as a "complete clarification of the principle." Research on the principle continues. Can we find colors made by layers in everyday objects?
6. How Can You See Color Changes in Nacre by Tilting a Shell?
Nacre is found inside the shells of many oysters, including Akoya and abalone. If you have a shell or a shell button, try tilting it slowly near a bright window. Does the color change with the viewing angle? Comparing the same spot to paper painted with paint makes it easier to see the difference between "painted color" and "light made by layers." Be careful not to cut your fingers on the edges of broken shells, and do not look directly at the sun. Research on interference colors is detailed in the 2021 Ehime University press release and studies on shell structure. The function of the pearl sac can be read in reports from the Mie Prefectural Fishery Institute [2].