Greyish agate with patches of blurry, foggy inclusions.Īn agate composed of multicoloured twisting and turning bands.Īn agate nodule partly filled with water.Īgate with concentric ring pattern, looking like an eye.Ī unique and rare variety of Fortification Agate from Fairburn, Custer Co., South Dakota, USA.Īgate with sharp-angled bands which resemble the outlines of fortifications of a castle.Īgate as a replacement material in fossils. Visit for gemological information about Agate.Ī variety of agate consisting of Jasper veined with Chalcedony.Ī variety of agate/chalcedony replacing coral.Ī variety of eye agate where the eyes are supposed to resemble the eyes of a bird.Ī pale blue banded variety of Agate (Chalcedony).Ī variety of agate from Botswana, banded with fine, parallel lines, often coloured pink blending into white.Ī naturally cemented matrix of broken agate fragments. These are listed as varieties of chalcedony, not as varieties of agate. Good examples are "feather agates" and "fire agates". There are a number of varieties of chalcedony that are called "agate" that do not match the definition given above. Roger Pabian's "Agate Lexicon" at UNL is a good source: Note: Agate is not simply "banded chalcedony." There are other types of chalcedony that are banded that do not match the description above banded flint, for example.īecause the colors and patterns found in agates are so varied and so characteristic for the respective localities, there is a confusingly large number of ever-changing varietal and trade names. When the difference in translucency or color between the layers is pronounced, agates with horizontal banding can be used for cutting cameos and engravings. Horizontal bands can serve as spirit levels to determine the original orientation of the specimen in the host rock. The banding consists of fine, irregularly spaced layers of small chalcedony spherulites and sometimes quartz crystals that precipitated in the cavity.
This type of banding is less common and usually accompanied by wall-lining banding. Horizontal Banding (also called Uruguay-type banding). Note that this type of banding is not restricted to walls of geodes: similar looking patterns of banding will develop around other structures that grew into the cavity, like crystals or moss-like inclusions.Ģ. In addition to the visible bands, there are compositional bands of varying trace element and hydroxyl concentrations on the micrometer scale (Frondel 1978, 1985). The changes in translucency reflect periodic changes of crystallite sizes and repetitive nucleation of new fibers at the growth front (Taijing and Sunagawa 1994 Cady et al 1998), as well as chemical composition (Frondel 1978 Heaney and Davis 1995). Since the chalcedony fibers grow from the walls to the interior of a cavity, a concentric, onion-like pattern develops. The individual bands run perpendicular to the orientation and growth direction of the chalcedony fibers. Two characteristic types of banding can be distinguished in agates (e.g. Because the individual crystallites in the fibers are not tightly interlocked, agate is slightly porous (e.g., Monroe, 1964), and can be dyed easily. In thin slices of agate, the fibers are sometimes visible in transmitted light and may cause interesting optical effects (see Iris Agate). Chalcedony fibers and concentric wall-lining banding visible in a thin slice of agateĪgate is made of fibrous length-fast chalcedony, sometimes with layers of quartzine (length-slow chalcedony) fibers (Michel-Lévy and Munier-Chalmas 1892 Correns and Nagelschmidt, 1933 Bernauer 1927 Braitsch 1957 Frondel, 1978 Flörke et al.
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