Agates occur world wide as crypto-to microcrystalline silica minerals especially deposited in various cavities such as in volcanic rocks. In the Thuringian Forest, a low mountain chain that is covered by the forest, agates and macroquartz are located at many sites, mostly in central cavities of larger rhyolithic spherulites. Figure 1 and Table 1 provide an overview to these sperulite-sites, which occur in rhyolitic volcanic sequences of Lower Permian (Rotliegend) age.
As a whole of the volcanic sequences, rhyolitic rocks are prevalent in the so-called Ilmenau-, Oberhof- and Rotterode-Formation of the Lower Rotliegend among the northeastern and southwestern borders of the Thuringian Forest. The effusive to subeffusive emplacement of these silica-rich volcanics was accompanied by the formation of a distinctive marginal facies. The specific petro-facial division of Oberhof Quartz Porphyries including the spherulite zone (ENDERLEIN and MÄDLER 1971) facilitates to find out the boundaries of the volcanic sequences in question. HOLZHEY (1993, 1994) and MEISTER (1988, 1994) extended and refined this facial division by taking into regard alteration processes within marginal parts of the rhyolitic volcanics.
According to BROWNE (1998), who points at investigations of "fluid-rock interactions" for silica-rich volcanics, the following summary regards hydrothermal alteration processes. As examples served those localities (cf. Table 1) which already can be distinguished in the field as spherulitic-perlitic rhyolites of the spherulite zone.
The rhyolitic spherulites, which contain an internal cavity (star-shaped or/and as lithophysal cavity), are infilled with quartz and other alteration minerals. The spherulites have diameters of a few centimetres up to more than 30 centimetres. Thuringian Forest agates, which are associated with different varieties of macroquartz, occur in a wide range of coloures and patterns (cf. Figures 2-10). Various shades of red are commonly displayed by these agates. However, the occurrences are minor in spread, and the agates are internally stressed. Stress fractures may appear after the agates have been sawn and polished.