Rhyolite

Rhyolite is an extrusive (volcanic) stone that formed on the surface and cooled down relatively rapidly. It appears in a plethora of colors and textures across the Trans-Pecos region, due to its complex geologic history. Abundant volcanism here during the middle Tertiary, about 47–18 million years ago, and subsequent erosion resulted in widespread outcrops and numerous varieties of this stone. Most examples from the region have medium to fine grained textures that can be recognized by the presence of phenocrysts, or mineral crystals, within their matrices. Since rhyolite fractures conchoidally, it was used as a source material to be made into stone tools. Fine-grained examples were often made into formal tools like dart or arrow points, while coarser textured varieties were used most often for expediency tools.

Arfedsonite (formally known as riebeckite or riebekite rhyolite) is a specific type of rhyolite found in abundance in the northwest part of Big Bend Ranch State Park (BBRSP). It outcrops in the Cienega Mountains and their foothills, but can also be found along a number of major and minor drainages in the area, as well as the gravel bars of the Rio Grande. Arfedsonite is a medium to coarse grained stone that is white or bluish-white in color and is readily distinguishable by splotches or bands of dark riebeckite crystals.

Because of its grainy texture afredsonite was primarily used by prehistoric peoples for expediency tools, rather than formal ones. This probably explains why relatively few quarry sites are present among the outcrops in the Cienaga Mountains. In the La Junta area, it is common in gravel bars along the Rio Grande and was often used for manos and other ground stone tools.

Source:

Mallouf, Robert J.
1993 Archaeology of the Cienga Mountains of Presidio, Texas. The Artifact 31(1):1-44.