Description
Agate is a unique natural wonder, with no two identical specimens: Each has a different pattern and color. It occurs in various volcanic and sedimentary rocks in nearly all countries on earth, with the most famous gems found in Brazil, India, and Madagascar (e.g., Ball and Burns, 1975; Breiter and Pasava, 1984; Priester, 1999; Moxon et al., 2006; Strieder and Heemann, 2006; Cross, 2008; Götze et al., 2009). Agates consist mainly of fibrous microcrystalline low-α quartz—i.e., macroscopic banded chalcedony (Götze, 2000). Graetsch et al. (1987), Heaney and Post (1992), and Heaney (1995) noted that in agates and other microcrystalline silica phases, low-α quartz commonly forms intergrowths with moganite, a monoclinic silica phase first described by Flörke et al. (1976, 1984) and approved as a new mineral by the International Mineralogical Association in 1999 (Grice and Ferraris, 2000)
Agates are a microcrystalline variety of silica, generally defined as banded chalcedony, but they also contain a variety of other silica polymorphs (Götze et al., 2001). The mechanism of rhythmic bands that produces agate is still not completely understood (Beaster, 2005). A number of explanations have been proposed for this banded texture and the rhythmic segregation of the silica polymorphs (see French et al., 2013). The prevailing hypothesis is that voids in volcanic rock are filled in cycles, from the rim to the center. Silica-rich fluids are believed to have penetrated the rocks through microfissures or infiltration canals. The shape and width of the zones and the general pattern of the agate all depend on the concentration of silica in the fluids, as well as the temperature, pressure, and timing of the fluid influxes. Other elements found in the fluids (Fe, Ti, Mn, and others) affect only the coloration of the agate zones. At the final stage of formation, mainly in the center of the void, idiomorphic quartz of different colors—brown, smoky quartz, amethyst, and rock crystal—can crystallize, sometimes accompanied by calcite, hematite, goethite, barite, and other minerals (Heaney, 1993; Götze et al., 2001; Moxon and Rios, 2004; French et al., 2013).
