Shades of indigo

Nature’s many hues of blue, in clear skies, calm seas, deep ice, alpine flowers1, juicy fruits2 and iridescent scales and feathers3, all derive from microscopic structures that scatter or filter light, or from chemically fragile pigments. Nature provides few sources of colourfast blue dyes for fabrics or durable blue pigments for inks, making indigo colourants from Indigofera plants or murex sea snails a prized commodity since at least 6,000 years ago4. Before indigo, the colour wheel for fabrics and inks had no cool side – none of the cyans, deep blues, violets, purples or magentas that abound in nature.

• Studies of flower colour have found altitudinal shifts in the Himalayas and Alps, from warmer yellows, whites and reds in valleys to cooler blues, violets and purples in alpine zones. High-altitude flowers typical of Asia, Europe and North America include gentians, delphiniums, campanulas and primulas.
• The blue-black fruits of heath barberry (Berberis empetrifolia) shine with a powdery glaucous bloom. On a stormy day in autumn, Pablo Neruda noticed them ripening along the rocky shoreline next to his coastal home in Central Chile. He was musing on identity and the limits to connectedness, while observing flocks of birds on migration to the warmer north:
  I watch the birds,
  arrowed like hungry ships
  flying over the sea
  in search of blue fire,
  in search of warm stone.
  …
  Here, I tell them:
  land, come to ground
  on the blue phosphorescence
  of bright indigo bushes
  and scatter the fruits of your flight
  along the coasts of Mexico.
  (Serenata de México, in Memorial de Isla Negra 1964,
  translated from Spanish by Alastair Reid).
• The electric blues of Morpho wing scales and peacock breast feathers function to display the male’s fitness to courted females, and perhaps also to rival males. Such costly signals – likely to attract predators as well as mates – may arise from natural selection on inherited preferences by females for particular features in males. They get passed down through the generations when the reproductive success won by their perceived beauty outweighs associated costs to survival. If his intended mate prefers blue, then the male had better show off his valour in iridescent blue, to ignite her desire to produce the most beautiful offspring.
• The Preceramic site of Huaca Prieta on the Pacific coast of Peru holds the earliest direct evidence of indigo dye for cotton fabrics, from 6,000 years ago. The dye was prepared from leaves of the Indigofera plant. Much earlier use of a Eurasian indigo plant, the woad Isatis tinctoria, is reported from one site in Georgia, where its leaves may have served medicinal or colourant purposes some 33,000 years ago. Palaeolithic uses of blue pigments include just one reported occurrence of copper mineral some 20,000 years ago in southern Siberia, and one reported occurrence of azurite mineral some 13,000 years ago in Germany.

C.P. Doncaster, Timeline of the Human Condition, star index