Agate ( ) is a banded variety of fibrous chalcedony. Agate stones are characterized by alternating bands of different colored chalcedony and often contain some visible quartz crystals. They are common in nature and can be found globally in a large number of different varieties. Most agates are concentrically banded, while some have bands that form parallel lines. Certain varieties of chalcedony without bands are commonly called agate (e.g. moss agate, fire agate), but these are not true agates. Moreover, not every banded chalcedony is an agate (e.g. banded chert).
Most agates form as nodules within volcanic rock, either as amygdules or thunder eggs. Agates also form in veins within any rock type and in silicified fossils. The processes that result in agate banding are not well understood; agate has never been made synthetically.
Agate has been popular as a gemstone in jewelry for thousands of years, and today it is also popular as a collector's stone. Many duller agates sold commercially are artificially treated to enhance their color.
Etymology The earliest known description of agate was by Theophrastus, a Greek philosopher and naturalist. According to both Theophrastus and the Roman naturalist Pliny the Elder, it was named for its discovery along the River Achates () in Sicily. However, etymologists believe the river was probably named after the stone, and the origin of the name is uncertain."Achate, N. (1), Etymology." Oxford English Dictionary. Oxford University Press. Retrieved 6 May 2026. The word agate first appeared in English in the 15th century, derived from the Anglo-Norman French agathe and the Latin achātēs.
Composition Agate is composed principally of chalcedony, a microscopic (microcrystalline) or submicroscopic (cryptocrystalline) form of silica. Most of the silica in chalcedony is quartz, while less than 5% to 20% is moganite, a quartz polymorph. All forms of silica, including quartz and moganite, have a chemical composition of . They differ in that quartz has trigonal symmetry while moganite is monoclinic. Over time, the less-stable moganite is converted to quartz; as a result, no moganite has been found in agates dating from before the Silurian period.
Unlike macroscopic (macrocrystalline) quartz, which is anhydrous, the chalcedony in agate normally contains small amounts of water, between 0.5% and 3%. Most is chemically bound in the form of silanol (Si–OH), with lesser amounts of molecular . Silanol presence decreases with age before levelling off at about 0.4% as a side effect of the conversion of moganite to quartz; this is a potentially useful indicator of age in younger agates. Macrocrystalline quartz is also present in most agates, particularly in the center as a solid core or geode, but also occasionally in bands.
Agate may also contain small amounts of opal, an amorphous form of silica containing up to 10% molecular water. Most is in the form of opal-CT, which consists of nanometer-sized crystals of the silica polymorphs cristobalite and tridymite and is therefore not truly amorphous. Fully amorphous opal-A is rare in agates. Like moganite, both varieties of opal are metastable and are eventually converted to quartz.
Impurities Agate often contains mineral inclusions or trace elements incorporated into the crystal structure. The most common trace elements present in agate are aluminum, calcium, potassium, sodium, and iron. Iron is responsible for producing the color of most agate bands; red and yellow bands are often colored by microscopic spheres of the iron oxides hematite and goethite, respectively. Manganese, chromium, and nickel may also be present in some colored bands. Some agates contain very small traces of uranium in the form of uranyl ions, causing them to fluoresce green under short-wave ultraviolet light. The most common mineral inclusions in agate are calcite and zeolites.
Structure Fibers
The chalcedony in agate is said to be fibrous, forming chain-like "fibers" of intergrown crystals. These fibers are twisted along their length, giving them a helical shape. Individual fibers were determined by X-ray diffraction to be 0.1-1.0 µm in diameter and up to several millimeters long. There are two different types of chalcedony fibers: length-slow (also known as quartzine) and length-fast. Agate primarily contains length-fast chalcedony fibers, consisting of crystals stacked perpendicular to the c-axis (side to side). Agate rarely contains quartzine, consisting of quartz crystals stacked parallel to the c-axis (tip to tip). Quartzine only occurs in the outer layer of agates or as thin intergrowths between layers of length-fast chalcedony fibers and macrocrystalline quartz.
Banding
Agates are broadly separated into two categories based on the type of banding they exhibit:
Wall banding occurs when agate bands roughly follow the shape of the cavity they formed in. In wall-banded agates, chalcedony fibers grow radially from the cavity walls inward, perpendicular to the direction of the bands. Level banding occurs when agate bands form in straight, parallel lines. In level-banded agates, chalcedony precipitates out of solution in the direction of gravity, resulting in horizontal layers of microscopic chalcedony spherulites.
Level banding is less common and usually occurs together with wall banding. Wall-banded agate is more fibrous than level-banded agate, which is more granular in texture. Consequently, wall-banded agate is denser and more compact.
Formation
Geologists generally understand the early stages of agate formation, but the specific processes that result in band development are widely debated. Since they form in cavities within host rock, agate formation cannot be directly observed, and agate banding has never been successfully replicated in the lab.
Nodular agate Agates are most commonly found as nodules within volcanic rocks. In mafic rocks such as basalt and andesite, they form inside cavities called vesicles (amygdaloids when filled), gas bubbles that were trapped inside the lava when it cooled. Since mafic lavas are poor in free silica, there are multiple theories of where the silica originates from, including micro-shards of silica glass from volcanic ash or tuff deposits and decomposing plant or animal matter. In felsic rocks such as rhyolite and rhyolitic tuff, agates form in spherulite-filled cavities called lithophysae, commonly called thunder eggs when filled with agate.
The cavities are filled with hot, silica-rich water from the surrounding environment, forming a silica gel. This gel crystallizes through a complex process to form agates. Agates are much harder than the rocks they form in; some varieties (e.g. Lake Superior agates) are frequently found detached from their host rock.
In mafic rocks, the vesicle walls are often coated with thin layers of celadonite or chlorite, soft, green phyllosilicate minerals that form from the reaction of hot, silica-rich water with the rock. This coating provides a rough surface for the chalcedony fibers to form on, initially as radial spherulites. The rough surface also causes agate husks to have a pitted appearance once the coating has been weathered away or removed. Sometimes, the spherulites grow around mineral inclusions, resulting in eyes, tubes, and sagenitic agates.
The first layer of spherulitic chalcedony is typically clear, followed by successive growth bands of chalcedony alternated with chemically precipitated color bands, primarily iron oxides. The center is often macrocrystalline quartz, which can also occur in bands and possibly forms when there is not enough chemically bound water in the silica gel to promote chalcedony polymerization. When the silica concentration of the gel is too low, a hollow center forms, called an agate geode. In geodes, quartz forms crystals around the cavity, with the apex of each crystal pointing towards the center. Occasionally, quartz in agates may be colored, occurring in varieties such as amethyst or smoky quartz.
Level bands usually form at the base of the vesicle or in the center when the gel stops adhering to the vesicle walls. This is probably due to a decrease in bound water in the gel.
Less commonly, agates can form as nodules within sedimentary rocks such as limestone, dolomite or tuff. These agates form when silica replaces another mineral, or silica-rich water fills cavities left by decomposed plant or animal matter.
Vein agate Agates can also form within rock fissures, called veins. Vein agates form in a manner similar to nodular agates (see above), and they include lace agates such as blue lace agate and crazy lace agate. Veins may form in either volcanic rock or sedimentary rock.
Fossil agate Fossil agates form when silica replaces the original composition of an organic material. This process is called silicification, a form of petrification. Examples include petrified wood, agatized coral, and Turritella agate (Elimia tenera). Although these fossils are often referred to as being "agatized", they are only true agates if they contain bands.
Variations Besides the two fundamental types of banding (wall and level), agates can exhibit a wide variety of band patterns, internal structures, and optical effects.
Band patterns Fortification agates are any wall-banded agates with sharp, well-defined bands. They are named for their appearance, which resembles the walls of a fort. Some authors do not make a distinction between fortification agates and other wall-banded agates.
Lace agates are vein agates that exhibit a lace-like pattern of bands with many swirls, eyes, bends, and zigzags.
Faulted agates have bands that were broken and slightly shifted by rock movement and then re-cemented together by chalcedony. They have the appearance of rock layers with fault lines running through them.
Brecciated agates also have bands that were broken apart and re-cemented with chalcedony, but they consist of disjointed band fragments at random angles. They are a form of breccia, which is a textural term for any rock composed of angular fragments.
Eye agates have one or more circular, concentric rings on their surface. These "eyes" are actually hemispheres that form on the husk of the agate and extend inward like a bowl.
Onyx is the traditional name for agates with level banding, although the formal definition of the term onyx refers to color pattern, not the shape of the bands. Accordingly, the name onyx is also used for wall-banded agates. Onyx is frequently misused as a name for banded calcite. The name originates from the Greek word for the human nail, which has parallel ridges. Typically, onyx bands alternate between black and white or other light and dark colors. Sardonyx is a variety of onyx with red-to-brown bands alternated with either white or black bands.
Internal structures Sagenitic agates, or sagenites, have acicular (needle-shaped) inclusions of another mineral, usually anhydrite, aragonite, goethite, rutile, or a zeolite. Chalcedony often forms tubes around these crystals and may eventually replace the original mineral, resulting in a pseudomorph. The term "sagenite" was originally a name for a type of rutile, and later rutilated quartz. It has since been used to describe any quartz variety with acicular inclusions of any mineral.
Tube agates contain tunnel-like structures that extend all the way through the agate. These "tubes" may sometimes be banded or hollow, or both. Tube agates form when chalcedony grew around sagenitic inclusions embedded within the agate, forming stalactitic structures. Visible "eyes" can also appear on the surface of tube agates if a cut is made (or the agate is weathered) perpendicular to the stalactitic structure.
Dendritic agates have dark-colored, fern-patterned inclusions (dendrites) that form on the surface of agates or in the spaces between bands. They are composed of manganese oxides or iron oxides.