Indigo

Indigo dye is an organic compound with a distinctive blue color. Historically, indigo was a natural dye extracted from the leaves of certain plants (usually Indigofera tinctoria), and this process was important economically because blue dyes were once rare. A large percentage of indigo dye produced today – several thousand tons each year – is synthetic. It is the blue often associated with denim cloth and blue jeans.

Indigo in The War Between the Provinces
Indigo was an important industrial crop in the northern provinces of Detina, and an indirect casus belli of the Detinan Civil War.