Agate
Agate is a variously translucent, banded variety of chalcedony. Agate stones are characterized by alternating bands of different colored chalcedony and may also include visible quartz crystals. They are common in nature and can be found globally in a large number of different varieties. There are some varieties of chalcedony without bands that are commonly called agate ; however, these are not true agates. Moreover, not every banded chalcedony is an agate; for example, banded chert forms via different processes and is opaque. Agates primarily form as nodules within volcanic rock, but they can also form in veins or silicified fossils. Agate has been popular as a gemstone in jewelry for thousands of years, and today it is also popular as a collector's stone. Some duller agates sold commercially are artificially treated to enhance their color.
Etymology
Agate was given its name by Theophrastus, a Greek philosopher and naturalist. He discovered the stone along the shoreline of the River Achates, now the Dirillo River, on the Italian island of Sicily, which at the time was a Greek territory.Composition
Agate is composed principally of chalcedony, a microscopic and submicroscopic form of quartz that grows in fibers. The chemical composition of quartz is, also known as silica. Normally, between 1% and 20% of the "quartz" in chalcedony is actually moganite, a quartz polymorph. Unlike macroscopic quartz, which is anhydrous, chalcedony normally contains very small amounts of water bound to its crystal structure.Agate contains multiple layers, or bands, of chalcedony fibers. The fibers are twisted, forming a helical shape. There are two different types of chalcedony fibers: length-slow and length-fast. Agate primarily contains length-fast chalcedony fibers, consisting of crystals stacked perpendicular to the c-axis. Some intergrown quartzine may also be present, consisting of quartz crystals stacked parallel to the c-axis.
Agate can sometimes contain small amounts of opal, an amorphous, hydrated form of silica. Agates also frequently contain macrocrystalline quartz, particularly in the center.
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.Agates are most commonly found as nodules within the cavities of volcanic rocks such as basalt, andesite, and rhyolite. These cavities, called vesicles, are gas bubbles that were trapped inside the lava when it cooled. The vesicles are later filled with hot, silica-rich water from the surrounding environment, forming a silica gel. This gel crystallizes through a complex process to form agates. Since agates usually form in lavas 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. Agates are much harder than the rocks they form in; some varieties are frequently found detached from their host rock.
In wall-banded agates, chalcedony fibers grow radially from the vesicle walls inward, perpendicular to the direction of the bands. 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-banded agates form when chalcedony precipitates out of solution in the direction of gravity, resulting in horizontal layers of microscopic chalcedony spherulites. [|Level banding] commonly occurs together with wall banding, often forming 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. Level-banded agate is less dense and less compact than wall-banded agate, as it is less fibrous and more granular.
Enhydro agates, or enhydros, form when liquid water becomes trapped within an agate nodule or geode, often long after its formation.
Agates can also form within rock fissures, called veins. Vein agates form in a manner similar to nodular agates, and they include lace agates such as blue lace agate and crazy lace agate. Veins may form in either volcanic rock or sedimentary rock.
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. Sedimentary agates also include fossil agates, which 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. Although these fossils are often referred to as being "agatized", they are only true agates if they contain bands.
Structural varieties
Agates are broadly separated into two categories based on the type of banding they exhibit. Wall banding, also called concentric banding or adhesional banding, occurs when agate bands follow the shape of the cavity they formed in. Level banding, also called water-level banding, ''gravitational banding, horizontal banding, parallel banding, or Uruguay-type banding'', occurs when agate bands form in straight, parallel lines. Level banding is less common and usually occurs together with wall banding.Wall-banded agates
Fortification agates are any wall-banded agates with tight, well-defined bands. They get their name from their appearance which resembles the walls of a fort. Fortification agates are one the most common varieties, and they are what most people think of when they hear the word "agate".Lace agates exhibit a lace-like pattern of bands with many swirls, eyes, bends, and zigzags. Unlike most agates, they usually form in veins instead of nodules.
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.
Sagenitic agates, or sagenites, have acicular 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 perpendicular to the stalactitic structure.
Dendritic agates have dark-colored, fern-like patterns that form on the surface or in the spaces between bands. They are composed of manganese or iron oxides. Moss agates exhibit a moss-like pattern and are usually green or brown in color. They form when dendritic structures on the surface of an agate are pushed inward with the silica gel during their formation. Moss agate was once believed to be petrified moss, until it was discovered the moss-like formations are actually composed of celadonite, hornblende, or a chlorite mineral. Plume agates are a type of moss agate, but the dendritic "plumes" form tree-like structures within the agate. They are often bright red or bright yellow. While dendrites frequently occur in banded agates, moss and plume agates usually lack bands altogether. Therefore, they are not true agates according to the mineralogical definition.
Iris agates have bands that are fine enough that when thinly sliced, they cause white light to be diffracted into its spectral colors. This "iris effect" usually occurs in colorless agates, but it can also occur in brightly colored ones.
Level-banded agates
Agates with level banding are traditionally called onyx, 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 also 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 with red-to-brown bands alternated with either white or black bands.Thunder eggs are frequently level-banded, however they may also have wall banding. Level banding is also common in Lake Superior agates.