The quandong’s fruit appear intensely blue, but without producing any pigment at all. Here’s this blue fruit evolved to use physics over chemistry.

“Blue Marble Closeup” by Daniel Ramirez (https://www.flickr.com/photos/danramarch/), CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en)

There’s a widespread belief in botany and food science that true blue fruits don’t really exist. Even those that have “blue” in their name (like blueberries or blue corn) are actually, on closer inspection, shades of purple, indigo or deep violet. They’re never a genuine blue. But deep in the rainforests of Australia and Southeast Asia grows a tree that breaks this rule entirely: Elaeocarpus angustifolius, better known as the “blue quandong,” “blue fig” or “blue marble” tree.

Its fruit is so vividly, impossibly cobalt that most people assume the color must have been digitally enhanced or chemically dyed. Yet if you ever came across one under natural sunlight, you’d see for yourself that the blue marble fruit gleams with a metallic, jewel-like brilliance. However, what makes this fruit so especially fascinating is that it doesn’t produce any blue pigment at all. Instead, its otherworldly hue comes from structural coloration.

Here’s how it conjures a color that nature rarely achieves through chemistry alone.

The Optical Illusion Hidden In The Blue Fruit Skin

The blue quandong grows throughout tropical Australia, Papua New Guinea, and parts of Indonesia. As research from Nature explains, it is a tall, spreading rainforest tree, with fruits that are approximately one to two centimeters (0.4 to 0.8 inches) in diameter. The fruits are small, relatively hard spheres, and their coloring is vivid even in dim lighting.

Researchers often describe the fruit as having perhaps the most intense natural blue hue found in the entirety of the plant kingdom. For most plants that are naturally blue, their coloration usually stems from anthocyanins: pigments that change color with pH. But as the authors of the abovementioned Nature study notes, when grinding the fruit in order to extract pigment, nothing blue appeared; the material they added to solution was actually a dull grey.

Quickly, the researchers discovered that the blue was a physical property of the fruit’s skin itself, rather than a chemical property.

As further research from Proceedings of the National Academy of Sciences explains, the secret behind the quandong’s blue lies in its multilayered cell wall structures, which are arranged in precise nanoscale layers. In turn, these layers are able to both reflect and reinforce certain wavelengths of light — in the same way that as the iridescent colors of butterfly wings, or the glimmering green-blue of peacock feathers.

But unlike insects or birds, which rely on iridosomes, the blue quandong’s structural coloration comes instead from alternating layers of cellulose fibrils, as well as air gaps. When observed under an electron microscope, it resembles a stacked deck of microscopic glass plates. And when this structure is repeated, it produces constructive interference. As a result, the fruit’s skin reflects only blue wavelengths, while simultaneously canceling out all others.

In simple terms, this means that the fruit isn’t blue in the same way blue dyes or paints are, as these rely on pigment. Rather, its microscopic architecture turns any white light that shines onto it blue. Most fascinatingly, it’s one of only six known fruits on Earth with this kind of structural coloration.

Why Did The Quandong Evolve To Have Blue Fruit?

Biologically, the color blue seems almost counterintuitive. Most fruits you’d find in the rainforest are red, orange or black, as they contrast ideally with the surrounding foliage in a way that signals ripeness. However, structural blue is incredibly rare, and that rarity itself may be its purpose.

As the abovementioned PNAS study explains, the most probable explanation is that the blue hue of the quandong fruit serves almost as a beacon for birds, who have exceptional color vision and sensitivity to blue and UV patterns. The possible evolutionary advantages of this include:

• Long-distance signaling. Given that it relies on its unique refraction of light, this means that the fruit is visible even under dense canopy shade. In turn, birds are easily able to spot it from afar.
• Ripeness cue. Unlike many other fruits, its blue persists even after it’s fallen off the Elaeocarpus angustifolius tree.

As a result, the blue quandong carved out its own niche by investing in long-distance visual marketing. Specifically, this evolutionary tactic makes the fruit especially attractive to birds — who, after eating the fruit, can disperse the fruits’ seeds far and wide.

Why The Quandong’s Blue Fruit Is An Evolutionary Outlier

To evolutionary biologists, the blue quandong is remarkable for more reasons than most would assume. Beyond its brilliant color alone, it also teaches us several invaluable lessons about evolution and materials science, which challenge many of our assumptions about plant and fruit coloration. Although structural color likely requires immense precision, it may still outweigh the chemical expensiveness of pigment-based coloration.

Although Elaeocarpus angustifolius may look like an unremarkable tree on its own, it still defies many of the beliefs scientists have long held about pigmentation — in a manner that seems almost like science-fiction. However, evolution often finds solutions that seem magical, until biology explains them.

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