The various minerals constituting green earth are presented: glauconites, celadonite, and chlorite. The characterization of green earth, its origin and optical and chromatic properties are discussed. Green earth was used rather late in antiquity and very rarely in mural painting. Only a pale green was available and local peculiarities may be noticed such as the use of the superposition of blue and yellow in Crete, exclusive use of copper-based green pigments in Egypt, and preponderant use of green earth in the Roman world. The examination and analysis of a green paint layer from the site of Lero, France, are described and compared with other Gallo-Roman mural paintings from Lyon and La Croisille-sur-Briance.