Lapis lazuli from Chile. (public display, Montana Bureau of Mines and Geology Mineral Museum, Butte, Montana, USA)

This blue material is from the famous Flor de los Andes Lapis Lazuli Deposit in central Chile. "Lapis lazuli" is an odd term for a rare, lazurite-dominated, metamorphic rock. The highest-quality examples on Earth are from the Hindu Kush Mountains of Afghanistan. The blue is lazurite, which is a sodalite-group feldspathoid mineral: (Na,Ca)8(AlSiO4)6(SO4,S,Cl)2 - sodium calcium chloro-sulfo-aluminosilicate-sulfate. The host rocks are marine fossiliferous limestone of Early Cretaceous age.

The rocks in this area of Chile were metamorphosed twice during the Tertiary: 1) contact metamorphism by intrusion of monzogranites of the Río Las Cuevas Granite (late Late Oligocene, 24 Ma); 2) sulfur-metasomatism by intrusion of dacites and rhyodacties of the Portezuelo del Azufre Intrusion (late Middle Miocene to Late Miocene, 9-13 Ma).

Neither of these metamorphic events converted the limestone to marble. The sulfur-metasomatism event resulted in the formation of blue-colored lazurite and brassy gold-colored pyrite.

Stratigraphy & age of host limestone: Río Tascadero Limestone, Berriasian Stage to possibly Barremian Stage, lower Lower Cretaceous