The Maya blue was studied by physical and chemical methods: optical microscopy, reflection spectrophotometry, x-ray and electron diffraction, electron microscopy, infrared absorption spectrophotometry, electrophoretic mobility, chemical stability, vat dyeing of attapulgite with indigo and their interaction on heating. A pigment at least similar to Maya blue is found to be produced by heating at 190°C a mixture of attapulgite and indigo. A set of pigments with variable indigo content was prepared in order to determine the limits of detection of the dyestuff. Infrared spectrophotometry was found to be the most suitable method for the identification of indigo. This dyestuff has definitely been identified in authentic samples of Maya blue. The conclusion that the colored principle in Maya blue is indigo is further supported by x-ray diffraction. Pigments similar to Maya blue containing indigo and attapulgite were also found in samples representative of other civilizations of ancient America: Mixtec, Toltec, and Aztec. The mechanism of the interaction of indigo and attapulgite is discussed. Electrophoretic mobility of the pigment indicates that indigo fixed on the clay behaves as a cationic additive on the dispersed electronegative colloidal attapulgite.