A bijou is a mixed alcoholic drink composed of gin, vermouth, and chartreuse.[1] This cocktail was invented by Harry Johnson, “the father of professional bartending”, who called it bijou because it combined the colors of three jewels: gin for diamond, vermouth for ruby, and chartreuse for emerald.[2] An original-style bijou is made stirred with ice as Johnson’s 1900 New and Improved Bartender Manual states “mix well with a spoon and serve.” This recipe is also one of the oldest in the manual, dating back to the 1890s.
The bijou was popular for several decades. Unlike the Manhattan and the martini, however, the bijou disappeared after Prohibition. It was rediscovered by “the King of Cocktails” Dale DeGroff in the 1980s, when he stumbled upon the recipe in Johnson’s book. While the original cocktail had equal parts of the three ingredients, DeGroff tripled the ratio of gin to vermouth and chartreuse to soften the taste profile. Eventually, his recipe became the standard
Reference: https://fr.wikipedia.org/wiki/Bijou_(cocktail)

This pre-Prohibition cocktail is a mixture of gin, sweet vermouth, and green Chartreuse—like a Negroni that took an abrupt turn two-thirds of the way through. The recipe originally called for an equal-parts mix of the three ingredients (plus a couple dashes of orange bitters), proportions that modern palates tend to find cloying. A more contemporary version of the recipe amps up the gin and lowers the green Chartreuse, rendering a more balanced drink.
The cocktail is attributed to bartender Harry Johnson, who included the recipe in the 1900 edition of his Bartender’s Manual and is said to have named it for the jewel tones of its component ingredients (“bijou” is French for “jewel”).

Référence:
https://www.liquor.com/recipes/bijou/