A distinctly banded fibrous chalcedony. Originally reported from Dirillo river (Achates river), Acate, Ragusa Province, Sicily, Italy.
The banding in agate is based on periodic changes in the translucency of the agate substance. Layers appear darker when they are more translucent (this may appear reversed in transmitted light). This effect may be accompanied and amplified by changes in the color of neighboring layers, due to other co-precipitated minerals. In old agates that have been subject to weathering and chemical alteration the differences in translucency may disappear, such specimens may turn almost opaque.
Agate 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. 1991; Graetsch 1994). In thin slices of agate the fibers are sometimes visible in transmitted light and may cause interesting optical effects (see Iris Agate). Because the individual crystallites in the fibers are not tightly interlocked, agate is slightly porous (e.g., Monroe, 1964), and can be dyed easily.
Two characteristic types of banding can be distinguished in agates (e.g. Graetsch 1994):
1. Wall-lining Banding. The individual bands run perpendicular to the orientation and growth direction of the chalcedony fibers. Since the chalcedony fibers grow from the walls to the interior of a cavity, a concentric, onion-like pattern develops. 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). In addition to the visible bands, there are compositional bands of varying trace element and hydroxyl concentrations on the micrometer scale (Frondel 1978, 1985).
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.
2. Horizontal Banding (also called Uruguay-type banding). This type of banding is less common, and usually accompanied by wall-lining banding. The banding consists of fine, irregularly spaced layers of small chalcedony spherulites and sometimes quartz crystals that precipitated in the cavity. Horizontal bands can serve as spirit levels to determine the original orientation of the specimen in the host rock. When the difference in translucency or color between the layers is pronounced, agates with horizontal banding can be used for cutting cameos and engravings.
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.
Because 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. Roger Pabian's "Agate Lexicon" at UNL is a good source:
Agates Lexicon http://snr.unl.edu/data/geologysoils/agates/agateslexicon.aspx
There are a number of varieties of chalcedony that are called "agate" that do not match the definition given above. Good examples are "feather agates" and "fire agates". These are listed as varieties of chalcedony, not as varieties of agate.
Visit gemdat.org for gemological information about Agate.
Physical Properties of Agate
Synonyms of Agate
Other Language Names for Agate
Varieties of Agate
A variety of agate/chalcedony replacing coral.
|Bird's Eye Agate|
A variety of eye agate where the eyes are supposed to resemble the eyes of a bird.
|Blue Lace Agate|
A pale blue banded variety of Agate (Chalcedony).
A variety of agate from Botswana, banded with fine, parallel lines, often coloured pink blending into white.
A naturally cemented matrix of broken agate fragments.
Greyish agate with patches of blurry, foggy inclusions.
|Crazy Lace Agate|
An agate composed of multicoloured twisting and turning bands.
An agate nodule partly filled with water.
Agate with concentric ring pattern, looking like an eye.
A unique and rare variety of Fortification Agate from Fairburn, Custer Co., South Dakota, USA.
Agate with sharp-angled bands which resemble the outlines of fortifications of a castle.
Agate as a replacement material in fossils.
Name proposed for a reddish agate with ovoidal patches of cacholong, etc.
An iridescent variety of agate - when sliced into a thin section it exhibits all the colours of the spectrum when viewed in transmitted light.
A colourful agate variety.
|Lake Superior Agate|
Believed to be the world's oldest agates, over 1 billion years old, these are found throughout the northern US having been spread from the original Lake Superior region by glaciation. It has generally pale colouring.
|Mexican Lace Agate|
Lacy or wavy agate from Mexico.
A variety of Agate (Chalcedony) containing inclusions of Pyrolusite.
In correct usage, the name refers to a black and white banded variety of Agate, or sometimes a monochromatic agate with dark and light bands (brown and white for example) - but traditionally the name was reserved for black and white banded agate, and brow...
|Pigeon Blood Agate|
A blood-red and white variety of agate from Utah.
According to Hey's 3rd Ed. this is 'a banded agate', which doesn't tell us much!
A variety of Agate with reddish-brown and either black or white bands.
Local name for agate or jasper coated by druzy quartz crystals.
|Quartz||282 photos of Agate associated with Quartz on mindat.org.|
|Amethyst||81 photos of Agate associated with Amethyst on mindat.org.|
|Copper||69 photos of Agate associated with Copper on mindat.org.|
|Calcite||44 photos of Agate associated with Calcite on mindat.org.|
|Chalcedony||36 photos of Agate associated with Chalcedony on mindat.org.|
|Carnelian||26 photos of Agate associated with Carnelian on mindat.org.|
|Goethite||15 photos of Agate associated with Goethite on mindat.org.|
|Hematite||15 photos of Agate associated with Hematite on mindat.org.|
|Smoky Quartz||13 photos of Agate associated with Smoky Quartz on mindat.org.|
|Thunder Egg||13 photos of Agate associated with Thunder Egg on mindat.org.|
References for Agate
Internet Links for Agate
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