A [cryptocrystalline] variety of [quartz], [chalcedony], has a waxy to dull luster and a tan, white, gray, or light-blue color. It is [translucent] but not [transparent]. Chalcedony does not have its own crystal shape but instead is found in masses that line or fill cracks, pores, and other cavities in rocks. It is formed when water containing silicon slowly seeps into these openings in the rocks and deposits the silicon dioxide there as chalcedony.
[Chalcedony] commonly occurs in some of the [Tertiary] rocks of the Gulf Coastal Plain. For example, chalcedony associated with [opal] is found near Freer in northern Duval County. In the [High Plains] of west Texas, it is found in alkali-lake deposits, such as at Shafter Lake in Andrews County and at Cedar Lake in Gaines County. In the Trans-Pecos country of west Texas, it can be found filling small cavities in [extrusive] [igneous rocks].
Polished [agate] from Rio Grande [gravels] of Zapata County, Texas.
A variety of [chalcedony] that generally is made up of more than one color is called [agate] (although agates consisting of several shades of a single color are also found). The colors may be spread out unevenly so that the agate has a cloudy appearance, or they can be arranged in wavy, in straight, or in concentric lines or bands. If the bands are straight and parallel, the specimen is called [onyx]. Agate that has a moss-like or tree-like design in it is called moss agate. Some agates make attractive gemstones when cut and polished.
[Jasper] from Uvalde County, Texas. Dark areas are brownish red; light areas are a yellowish-tan.
Much [agate] has been found filling cavities in [Cenozoic] [igneous rocks] in Brewster, Presidio, and other counties in the [Trans-Pecos] country of west Texas. It has been found also in an area about 10 to 15 miles wide along the Rio Grande, mostly in southern Webb County and in Zapata and Starr counties.
Trees and other plants have been replaced by [agate]. Many specimens of [agatized wood] have been collected from [Tertiary] [formations] in Fayette, Gonzales, Lee, Washington, and other counties of the [Gulf Coastal Plain]. (The agatized wood, along with opalized wood, occurs within about 20 miles of the boundary between no. 2 and no. 3 on the [geologic map], pp. [4]-5.)
A hard, compact, slightly [translucent] variety of [cryptocrystalline] [quartz] is called [jasper]. It commonly has a red, brown, or yellow color due to the presence of an iron oxide, such as [hematite]. Some jasper is made up of irregular bands of more than one of these colors. This variety of quartz often is polished to make attractive gem or ornamental stones. It has been collected at several localities in Texas, particularly from creek and river [gravels]. Starr and other nearby counties along the Rio Grande have furnished a number of good specimens.