What natural pigment was used for the blue turban?

The natural pigment used for the blue turban was ultramarine.

Vermeer’s ultramarine was made from lazurite, the blue mineral found in lapis lazuli. In the seventeenth century, artists obtained high-quality lapis lazuli mainly through long-distance trade from Central Asia, making natural ultramarine one of the most expensive pigments available. Its intense, slightly violet-blue color helped give the headscarf its striking brilliance.

Scientific examination of Girl with a Pearl Earring found bright ultramarine particles in the blue headscarf. Vermeer used the pigment in more than one way: the scarf contains a mixture of ultramarine and lead white, while a thin layer of relatively pure ultramarine helped enrich the surface color. He also mixed ultramarine into parts of the girl’s yellow clothing.

A common mix-up is cobalt blue, which was not introduced as a widely used artists’ pigment until the nineteenth century. Prussian blue was also developed later, around the early eighteenth century, while azurite is a different copper-based blue mineral.