“"Pecos Diamonds" are quartz crystals that are found along the Pecos River valley, either weathered out of or still inside a selenite rock matrix of the Permian Seven Rivers formation in south-eastern New Mexico. They are authigenic quartz crystals that grew inside the matrix and not in a pocket. They come in different shapes and habits, this one is a "pseudocubic" quartz that is dominated by very large r-faces. It is not really a cube because the angles between the rhombohedral faces are not 90Â° but 85.2Â° and 94.8Â°, respectively, so the "cube" is slightly skewed. The edges of the crystal are 10-15mm long. Found and donated by John Kashuba.