Glass Mountain is a large, exposed mound of selenite crystals. Selenite is a variety of gypsum (CaSO4•2H2O) in the form of glassy crystals. Gypsum is a common mineral found in the sedimentary rocks of this area.
The crystals of glass mountain are somewhat unusual in size and in the massiveness of the deposit. Glass Mountain formed as a result of groundwater flowing through the Entrada Sandstone. This water carried dissolved gypsum, which started to crystallize, forming what has been called a "gypsum plug". This plug is now being exposed as the soft Entrada Sandstone erodes away.
As you visit Glass Mountain, please remember that collecting of any kind is prohibited in all national parks.