The fruit of the African marbleberry (Pollia condensata) is a true living gemstone, possessing a stunning metallic blue luster that never fades.

       But the blue color of these berries isn’t quite what most of us imagine. At least, they don’t contain any blue pigment. This cool hue is the result of a subtle optical illusion that can only be seen upon close examination of the fruit’s cells under a microscope.

       A team of researchers from the University of Cambridge in the UK has done just that, discovering how this marbled berry gets its distinctive appearance.

       The world of color we see is usually the result of subtractive color mixing. Materials absorb different wavelengths of white light; the remaining light forms the object’s color.

       This fruit uses structural dyeing techniques where the fibers on its outer cell walls are arranged in a special twisted structure that causes wave interference.

       This superposition method means that some wavelengths cancel each other out while others are enhanced, creating a unique rainbow effect in certain parts of the spectrum. In this particular case, the blue wavelengths are primarily preserved.

       The researchers wrote: “The bright blue color of this fruit is more saturated than the blue color of many previously described biological materials.”

       ”This is the highest reflectivity ever recorded in a terrestrial organism, surpassing even the deep blue of a beetle’s exoskeleton, a bird’s feathers, and the famous blue scales of the Morpho butterfly.”

       There are many examples of structural coloration in nature, but it is rare in fruits. The fruits of Elaeocarpus angustifolius also exhibit similar structural coloration, although their luster is not as good as that of other fruits.

       Compared to light reflected by a silver mirror, marble berries reflect 30% of the incident light, demonstrating exceptionally high reflectivity. While the twisted layers of fibers allow blue light to predominate, they also blend in some other colors, ultimately resulting in a slightly pixelated visual effect.

       The researchers wrote: “Our study shows that variations in the thickness of multiple layers in the fruits of the Polya plant produce an optical response that is clearly unique in nature.”

       ”Although the blue reflective effect predominates, the sparse distribution of green and red reflective cells gives the fruit a fascinating pixelated (pointillist) appearance that has not been documented in any other organism.”

       Researchers believe there is an explanation for this: by attracting birds with their bright appearance, the fruits of P. condensata promote wider seed dispersal and allow the plant to continue to survive.

       Since this berry has no nutritional value, this seeded fruit can only attract attention for its attractive appearance.

       Peacock feathers also use similar techniques to attract attention, but they use a different method of structural coloration in combination with pigments.

       Millions of years of evolution have once again finely and elegantly tuned nature—a subtlety that takes your breath away even before you understand its secrets. We still lag behind in developing our own colors and materials.

       ”This inconspicuous little plant has found a brilliant way to send an irresistible, bright, colourful, iridescent signal to every bird in the area, without wasting its precious photosynthetic reserves on foraging for birds,” said Beverley Glover, a plant scientist at the University of Cambridge.