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SRI Petrography ID: 234


[de] Achat
[en] agate


Agates are rhythmically banded, straticulated aggregates of quartz, which are composed micro-to crypto-crystalline, fibrous and also radiating. Agates consist mainly of chalcedon layers and some other varieties of SiO2, which differ in their crystallographic orientation. As agate geodes they often enclose cavities in volcanic rocks, whereas agate almonds fill them all. The fracture is conchoidal showing resinous luster. Density and refractive index are slightly lower than those of pure quartz, due to embedded submicroscopic cavities.


almost colorless, light blueish, yellowish, grey, greenish, reddish to deep red, brown, white or black; staining caused by foreign mineral inclusions of predominantly Al-silicates, iron oxides, calcium carbonate

Grain Size

no visible grains without using a microscope, micro-to cryptocrystalline, microfibrous


massive, dense, parallelfibrous, banded


fine-to cryptocrystalline quartz, SiO2 varieties as quartzine, fine quartz, moganite and lutecite; as the last, youngest segregation in geodes are common macrocrystalline quartz or amethyst;


worldwide occurrence


Even the Sumerians, Assyrians, Egyptians and Mycenaeans used agate and its varieties for the production of jewelry, gems, cameos and decorative art objects. Some possibilities for artificial coloration, which is due to the porous structure, were already known.
Because of its great tenacity agate is also used as a raw material for the manufacture of jewels in the watch industry and precision engineering, the production of ball mills, mortars, pestles, polished stones and numerous other items.
Moss agate or mocha stone is a chalcedony containing small dendrites (tree-like growths), which consist of iron oxide or an iron- rich chlorite
Onyx or sardonyx are flat banded varieties, onyx having white and grey, brown or black bands and sardonyx having white, or blueish white and red, or brownish red bands.




Broader Terms

non clastic silica-rich sedimentary rocks