Elaeocarpus angustifolius, or blue quandong, is a rainforest tree found in eastern Australia. It is quick to grow, reaching a height of 50 meters, and performs a key role in regenerating the rainforest after disturbance. In addition to the intriguing qualities shown in the photographs, this species also has beautiful fringed white flowers and stately buttressed roots.
The fruits of Elaeocarpus angustifolius, writes David W. Lee in a paper (PDF) published in Nature (Ultrastructural basis and function of iridescent blue colour of fruits in Elaeocarpus), achieve their colour in a very unusual way. While nearly all blue plant parts contain a pigment, the fruit of the nearly 60 species of the Elaeocarpus appear blue because their cuticles reflect blue light, in much the same way that peacocks (Pavo cristatus L.) and some beetles appear blue. Key to the iridescent blue qualities of Elaeocarpus fruit is the iridosome, a layer beneath the fruit’s outer cell wall that consists of parallel rows of translucent strands of just the right thickness and spacing to reflect only blue light. Lee posits that the evolutionary advantage of iridescent colour (as opposed to pigmented colour) are twofold: first, the brilliant blue colour is highly visible against a backdrop of green foliage, even persisting after the carbohydrate-rich mesocarp has been consumed. Second, most fruits are only able to photosynthesize when green, but by achieving colour through iridescence, the fruit of Elaeocarpus are able to continue to contribute to the plant’s carbon economy even when fully ripe.
Another, unrelated species that has iridescent blue berries is Pollia condensata. This article in Wired explains how the “natural world’s most intense colour” is produced. It seems the principles behind Pollia condensata‘s fruit colour are similar to the blue quandong.