Agate ‒ a spectacular form of SiO2 and a famous gemstone ‒ is commonly characterized as banded chalcedony. In detail, chalcedony layers in agates can be intergrown or intercalated with macro-crystalline quartz, quartzine, opal-C, opal-CT, cristobalite and/or moganite. In addition, agates often contain considerable amounts of mineral inclusions and water as both interstitial molecular H2O and silanol groups.
Agates belong to the most fascinating mineral objects in nature because of their wide spectrum of colors and spectacular morphologies. Therefore, they play a dominant role as gemstones and cut stone since antiquity. The name “Agate” can be dated back to ca. 350 B.C. (Theophrast) and was probably related to the discovery of agates in the river Achates (recently Drillo) in Sicily. Today, agate deposits and agate treatments are known from historical and recent sites all over the world.
In general, agate occurrences are distributed around the world on all continents, and agates have already been formed very early in the Earth’s history. The oldest known occurrence, the Warrawoona agate in Western Australia, was found in 3.48 Ga old metamorphosed rhyolitic tuffs. More than one billion years old agates are also known from the basalts of the Lake Superior region in the USA and Canada.
A closer view reveals that agate occurrences are in particular connected with geological periods of strong volcanic activities such as huge basaltic lava flows or eruptions of acidic lava from Permian to Tertiary. Chemical and mineralogical analyses of the host rocks show that most global agate occurrences are related to both SiO2-poor (andesites, basalts) and SiO2-rich (rhyolites, rhyodacites) volcanic rocks (Figure 1a-d).
However, agates can also be formed by other processes and in other rock types. In different host rocks, hydrothermal vein agates occur in mm- and cm-sized fissures and veins (Figure 1e), but can also reach thicknesses of several dm and lengths of several hundreds of meters. In addition, agates of sedimentary origin have been found as irregular forms in stratigraphic sequences of carbonate rocks and clastic sediments (Figure 1f) including silicification of residues of animals and plants in the surface region of certain sediments and volcano-sedimentary units. Another common feature is the secondary deposition and redistribution of agates from primary deposits. Agates in clastic sediments (river gravel and marine sediments) are known from the surroundings of many agate occurrences worldwide (Figure 1g).