An interesting book that looks at the author's fascination with the colour blue. He travels the world, talking to scientists and others, and gives a look at various aspects of blue: from objects with the colour to how we look at and react to blue.

• "Stones" looks at how blue appears in stones and combinations of minerals. Lapis lazuli is known to be blue since ancient times and used for decorations. The Egyptians eventually figured out how to make Egyptian blue tiles. By grinding, heating and combining lapis lazuli with other substances, ultramarine blue was produced, a colour more valuable than its weight in gold. The act of alchemy would produce the next blue substance, Prussian blue. In modern days, a vivid blue would be accidentally created: YInMn Blue. And maybe one day, the mineral ringwoodlite that makes up much of the mantle is the earth could give another way to make blue.

• "Seeing" looks at how we see colour, especially blue. Starting with the electromagnetic spectrum of visible light, he goes into the fine cells in eyes that let us see colour and why we see colour better than other mammals, but worse than birds and insects. He then shows how other cells in the eye are affected by blue light (even blind people), which is why blue light may set our daily rhythms, hence the movement to reduce blue light at night. A look at colour illusions (like the infamous blue dress image) are given. A brief look at the philosophical question of why we 'see' blue is also given.

• "Plants" looks at how plants produce blue. From the mystery of why plants are green, the author then shows the challenges of producing blue flowers in plants. People in the past (and chemistry researchers today) have looked at pigments produced by plants to colour clothes and food. While red and yellow pigments are common, pigments for blue are rare and highly sought after. Producing a blue rose is also a target that is yet to succeed (although blue chrysanthemums have been produced). In the past, indigo was a popular blue dye, but now artificial dyes are common.

• "Speaking" looks at how language affects how we refer to blue. Gladstone once had a theory that ancient Greeks couldn't perceive blue to explain why the word for blue is not used. But further studies would show that it is an effect of how language develops words for colours instead. Other studies would show that in a small way, having words to refer to different shades of blue can have an effect on whether people can more easily see shapes and shades of blue in experiments.

• "Animals" looks at how animals use blue. Besides pigment, animals produce blue through the careful creation of structures that mainly reflect blue light. Harder to explain was why animals use blue, or colours in general. It was initially thought that colour was used just for camouflage. Later on, the idea that colours are used as a warning signal emerged. Then came the idea that colour is also used as a signal for fitness in mate selection. Unfortunately, humans preferences and desires for blue animals have driven some species to the edge of extinction.