The Piedra Lumbre Formation is a Precambrian geologic unit exposed in the Picuris Mountains of northern New Mexico, dated to about 1,475 million years ago in the Calymmian period. Composed chiefly of phyllite, schist, quartzite, and phyllitic slate, it has been so intensely deformed within the Hondo syncline that its original layering is hard to trace. Its age and relation to nearby rocks have made it important to geologists, helping support removal of both the Pilar and Piedra Lumbre formations from the Hondo Group and strengthening evidence for the ancient Picuris orogeny. Its classification evolved from the 1950s through its formal naming in 1976.