Like a magical pearl of the forest, these stunning irridescent blue berries look like something out of a fairytale. Their shimmering metallic sheen have earned them the name 'marble berry'. Yet despite their eyecatching apprearance, any magical qualities that they may possess are yet to be discovered. However, the secret of their amazing colour really is very interesting...
The blue colours that we usually see in nature- in flowers like delphniums, bluebells and agapanthus, and in fruits like blueberries, are caused by pigments called anthocyanins. These compounds are also responsible for the pinks and purples like those found in beetroot and red cabbage.
Pigments (including those that we use to paint with) absorb certain wavelengths of light. The wavelengths that are not absorbed by the pigments are the colours that we see.
In the case of these African Pollia berries, there are no pigments at all present in the berry! The appearance is entirely due to something called structural colour. This means it is the physical structure in the cells of the berries that causes the colour that we see. In the case of this berry, there are spirals of cellulose microfibrils which bounce back the reflected light in a certain way.
Because the colour is caused by a physical structure, rather than an organic pigment which degrades over time, the intense blue colour of these berries persists while the green pigments in the leaves and stalks fades. The image below is from a herbarium specimen from the collections at the Royal Botanic Gardens, Edinburgh.
If we take a closer look at the label on this herbarium specimen, we can see that it was collected in Cameroon in September 1895. Thats over 120 years ago!
Although uncommon, this phenomenon is not unique in the natural world - it is more usually found in butterflies and beetles.
I'm not sure I would know where to start if I were to paint these glorious little jewel like fruits, but here is a wonderful example of structural colour from a creature of the entomological world by the wonderful Natural History Illustrator Lizzie Harper.