Agate

A distinctly banded fibrous chalcedony which forms in various rocks (mainly volcanic ones, but also in sediments), at temperatures between ca. 20 and 200 °C.

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 colour of neighbouring 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) fibres (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 fibres are sometimes visible in transmitted light and may cause interesting optical effects (see Iris Agate). Because the individual crystallites in the fibres 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 fibres. Since the chalcedony fibres 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 fibres at the growth front (Lu 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 colour 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 colours 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.

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