Agate

Agate is a banded, concentric shell-like chalcedony, sometimes contains opal . The fine quartz fibers are oriented vertically to the surface of the individual band layers. The bands can be multicolored or of the same color. The Agates of the exhausted German mines had soft to strong colors (especially pink, red, or brownish) and were separated by bright white bands. The south American Agates are mostly dark grey and without special markings;only through dyeing do they receive their lively colors. (see image below).

Transparency of Agates varies from nearly transparent to opaque. In thin slabs, even the opaque Agates are mostly translucent. The name Agate is supposedly derived from the river Achates (now called the Drillo) in Sicily.

Origin: Agates are found as ball- or almond shaped nodules with sizes ranging from a fraction of an inch to a circumference of several yards; more rarely they are found as fillings of crevices in volcanic rocks (such as rhyolite and dacite). The bands are though to be formed by rhythmic crystallization, but scientific opinions vary as to how. It was thought that the agate bands crystallize gradually in hollows formed by gas bubbles from a siliceous solution. Recently the theory that their formation is simultaneous with that of the matrix rock has won support. According to this idea, the liquid drops of the silicic acid cool with the cooling rock and produce a layered crystallization from the outside. A new theory postulates that rather than the liquids penetrating the agate walls, colloid solutions, i.e., substances with very fine sizes of grains, flow into the agate hollows. The various agate bands vary in thickness, but normally their thicknesses remain constant throughout the nodule. Where the inner cavity of the nodule is not filled with agate mass, well-developed crystals may have formed: rock crystal, amethyst, and smoky quartz; sometimes accompanied by anhydritspat, ankerit, baryite, calcite, Goethit, hematite, siderite, and zeolite. A nodule with crystals in the central cavity if called a druse; if the inside is completely filled, one speaks of a geode.

Agate Varieties:

Eye Agate: Ring shaped design with point in the center similar to an eye. A type of orbicular agate.

Layer Agate: Layers/bands of about the same size parallel to the outer wall of the agate nodule.

Dendritic Agate: Colorless or whitish, translucent chalcedony with dendrites.

Enhydritic Agate: (Also called enhydro or water stone) Agate nodule or mono-colored chalcedony nodule, partly filled with water which can be seen through the walls. After the agate is taken surrounding rock, the water often dries out.

Fortification Agate: Agate bands with jutting-out corners like the bastions of old fortresses.

Fire Agate: Opaque, limonite-bearring layered chalcedony with iridescence which is created through diffraction of the light by the layered structure.

Orbicular Agate: Circles of the agate layers, arranged concentrically or excentrically around a centerpoint.

Moss Agate: Translucent chalcedony with moss -like inclusions of hornblende or chlorite.

Scenic Agate: Shows landscape like images through dendrites.

Pseudo Agate: (also called polyhedric quartz). Interior similar to Agate with layering and druse opening, although outside not nodule-like, but geometric shape. Formed probably as wedge-filling of crystals, which later were loosened. Occurrences in loose rocks in Brazil. Individual pieces can measure up to 28 in (75 cm). Many fanciful names.

Tubular Agate: Agate with numerous tubes (old feeding canals). The mouth opening of the canals is usually bordered concentrically.

Thunder egg or Sandstone Agate: Layered agate nodule with strongly furrowed outer surface. Deposits in Oregon (United States), Australia, and Mexico.

Brecciated Agate: Agate, broken, but cemented together by Quartz.

Deposits: The most important Agate deposits at the beginning of the 19th century in the neighborhood of Idar-Oberstein/Rheineland-Palatinate, Germany. These have now been worked out. Nodules were found as large as human heads with beautiful colors of grey, pink, red, yellow, brown, and pale blue. These could not be dyed. The most important deposits now are in the south of Brazil (Rio Grande do Sul) and in the north of Uruguay. The deposits are layered in weathered materials and river sediments, and are derived from melaphyric rocks. The color is generally grey; the striations are hardly recognizable. They can be given an attractive appearance by dyeing. Other deposits are in Australia (Queensland), China, India, the Caucasus, Madagascar, Mexico, Mongolia, Namibia, and the United States (Wyoming and Montana).