A variety of quartz with a milk-white color and a glassy to greasy luster is called _milky quartz_. It occurs either as crystals or as crystalline masses. Very little light will pass through it. In central Texas, milky quartz occurs abundantly in the Precambrian rocks of the Llano uplift area in Blanco, Burnet, Gillespie, Llano, and Mason counties. It also is found in some of the rocks of the Trans-Pecos country of west Texas, such as in the Carrizo Mountains of Culberson and Hudspeth counties. Other good places to look for this variety of quartz are in the sands and gravels along many streams in Texas. Some quartz has a glassy to a greasy luster and a rose or pink color. _Rose quartz_, as this variety is called, commonly occurs as masses rather than as individual crystals. It can be found along some of the streams in Texas and also in igneous rocks, such as those of the Llano uplift area of central Texas. A kind of quartz with a smoky brown, a smoky yellow, or a dark brownish-black color is called _smoky quartz_. Its luster is glassy, and it may be either translucent or transparent. Smoky quartz is commonly found as crystals that are shaped like 6-sided prisms with pyramid-like ends. It is commonly associated with igneous rocks, and beautiful specimens have been found in the Lake Buchanan area of Llano and Burnet counties in central Texas. [Illustration: Smoky-quartz crystals from Burnet County, Texas.] 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. [Illustration: 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.