CRYPTOCRYSTALLINE VARIETIES
Chalcedony.—When free from impurities of various oxides and other compounds, chalcedony has little to render it pleasing as a gemstone. It is mostly gray, white, brown, or bluish and commonly has a waxy luster. Some of the chalcedony found along the Rio Grande Valley and in west Texas will take dyes, and local lapidaries have had some success in dyeing this material various shades of blue, green, yellow, and red. When the chalcedony is naturally colored and variegated, usually in bands, mossy figures, or dendritic forms, it is called agate.
Agate (variegated chalcedony).—The wide variety of markings and colors available together with the ease of cutting make agate a favorite of many lapidaries. Fine agate has been found at numerous localities in west and south Texas. Fine plume agate, famous throughout the United States, is found south of Alpine. Plume agate is characterized by dendritic or tree-like inclusions and is mostly cut into very handsome cabochons. The agate from south of Alpine commonly contains black, red, yellow, or brown plumes within the same piece. The variety of colors and lack of porosity of this agate make it highly desired among lapidaries. The agate occurs loose on the surface of the ground and in the soil in small nodules that have a very rough, brownish surface. These nodules are mostly less than 3 inches in diameter, although specimens of gem quality have been found that exceed 200 pounds.
Some very fine agate has been found in the vicinity of Needle Peak, Presidio County. This material is mostly green moss agate in clear chalcedony and commonly contains small yellow “sun-burst” figures. The contrasting yellow and green design makes very beautiful cabochons.
Fine agate has been found south of Marfa, Presidio County. This agate is mostly clear chalcedony with black, yellow, or variously colored plumes, moss, or “bouquet-like” figures.
Numerous other localities in Presidio and Brewster counties have produced good agate.
Various amounts of agate, jasper, and chalcedony occur in the gravels of the Rio Grande in varying quantities from Big Bend National Park downstream to Brownsville. This agate is found both in the present river gravels and in the older river gravels that now are located on nearby hills and slopes up to several miles north or south of the present Rio Grande. The greatest concentration of agate and related gem materials seems to be in the area between Laredo and Rio Grande City. Vast quantities of excellent gem material have been removed from this area for many years ([Pl. IV]). The agate occurs as rounded, stream-worn cobbles and commonly has a thin white coating that makes it difficult to distinguish from the abundant chert and other rocks. The agate occurs in cobbles that are mostly 3 to 6 inches in diameter, but specimens of gem quality that exceed twice this size are known. The agate varies greatly in design and color. Plume, moss, banded, and sagenitic agate occur in these gravels in a wide variety of colors. The jasper in the Rio Grande gravels is yellow, red, green, or various shades of these and is commonly suspended as angular fragments in clear chalcedony.
Good agate has also been found near Balmorhea in Reeves and Jeff Davis counties and in smaller amounts at numerous other west and south Texas localities.
Agatized wood (see Fossil wood, [pp. 20-21]).
Carnelian (translucent reddish chalcedony).—This variety of chalcedony in small quantities has been reported from near Van Horn, Hudspeth County. Small pieces of carnelian have been found in the gravels of the Rio Grande, but finds have been few and scattered.