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 no