Ogallala quartzite

Ogallala quartzite is not a true metamorphic quartzite. It is sand and gravel of the Ogallala Formation that groundwater cemented with opal and a little chalcedony. Kansas Geological Survey work found it in lenses from a few feet to over half a mile long. North of Texas the colour runs from greenish gray to greenish brown to bright green, weathering to light gray, white or reddish brown. In west Texas it is pink to gray-pink. Texture runs from fine sand to coarse conglomerate. Well-cemented pieces are dense and tough, break through the grains rather than around them, and can show a subconchoidal fracture. Pieces with many pores are weaker. The Survey calls the name quartzite a convenience, used by local people and builders.

Silicified sand and gravel in the Ogallala Formation. Silica, mostly opal with a little chalcedony, cements the grains. Lenses sit within loose sand and gravel and often rest directly on silicified Niobrara chalk. The formation was sampled at 27 localities in Kansas plus sites in Texas, Colorado, South Dakota and Nebraska.