What’s your favorite color? Probably blue, right? Everyone like it–five of Pantone’s last 11 colors of the year have been shades of blue, including this year’s pick, turquoise. A Cambridge psychological study showed even rhesus monkeys pick blue as their favorite (green was second). According to Pantone, blues are calming, inviting, and hopeful, which is why they’re so common these days. But did you know that blue pigment used to be more valuable than gold?

Joshua Cohen traces the history of what was once the most valuable color in his fascinating–and exhaustingly researched–essay “Thirty-Six Shades of Prussian Blue,” in Triple Canopy‘s latest issue. The first blues came from rocks and plants and were either unreliably color-fast, or way too expensive. Ultramarine was so rare when it first became available that it “was said to have once been more expensive than gold, and Renaissance artists had to negotiate with their patrons for individual drops.” Then in 1704, Heinrich Diesbach, a Berlin chemist, mixed some cochineal (a natural red dye) with iron sulfate and a cyanide mixture, and discovered Prussian blue, the first synthetic color.

Prussian blue was an instant sensation. Its manufacture escaped regulation by painters’ guilds since it was considered a chemical and not paint, and its use quickly spread. Cezanne’s mustache was stained with it, Ruskin hoarded it, it was Wordsworth’s favorite color, E.E. Cummings and Baudelaire wrote about it. It was used as an invisible ink in World War II (letters would be written in iron sulfate then sprayed with the cyanide solution) and the FDA even says it can be used to get rid of some radiation poisoning. Crayola changed their Prussian Blue crayon to Midnight Blue in 1958 because they realized no one knew what Prussia was, but now you can know more than you ever thought possible about the color that bears its name.