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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 (moss agate, fire agate, etc.); 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 Silicification fossils. Agate has been popular as a gemstone in Jewellery 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.
Agate contains multiple layers, or bands, of chalcedony fibers. The fibers are twisted, forming a helix shape. 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). Some intergrown quartzine may also be present, consisting of quartz crystals stacked parallel to the c-axis (tip to tip).
Agate can sometimes contain small amounts of opal, an amorphous, hydrated form of silica. Agates also frequently contain macrocrystalline quartz, particularly in the center.
Agates are most commonly found as nodules within the cavities of volcanic rocks such as basalt, andesite, and rhyolite. These cavities, called vesicles ( amygdaloids when filled), are gas bubbles that were trapped inside the lava when it cooled. (2026). 9781591932826, Adventure Publications. ISBN 9781591932826 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 (e.g. Lake Superior agates) 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 Mineral variety 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 agate, or enhydros, form when liquid water becomes trapped within an agate (or chalcedony) 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 (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 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 ( Elimia tenera). Although these fossils are often referred to as being "agatized", they are only true agates if they contain bands.
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. (2026). 9781591932826, Adventure Publications. ISBN 9781591932826 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. (2026). 9781591932826, Adventure Publications. ISBN 9781591932826
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. (2026). 9781591932826, Adventure Publications. ISBN 9781591932826 These "tubes" may sometimes be banded or hollow, or both. Tube agates form when chalcedony grew around sagenitic inclusions embedded within the agate, forming Stalactite 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-like patterns (dendrites) that form on the surface or in the spaces between bands. (2026). 9781591932826, Adventure Publications. ISBN 9781591932826 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 group 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 (from inclusions of hematite) or bright yellow (from inclusions of goethite). 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 Diffraction into its spectrum colors. This "iris effect" usually occurs in colorless agates, but it can also occur in brightly colored ones.
Thunderegg are frequently level-banded, however they may also have wall banding. Level banding is also common in Lake Superior agates. (2026). 9781591932826, Adventure Publications. ISBN 9781591932826
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(2026). 9781591932826, Adventure Publications. ISBN 9781591932826 |
Industrial uses of agate exploit its hardness, ability to retain a highly polished surface finish and resistance to chemical attack. Historically, it was used to make bearings for highly accurate laboratory balances and mortars and pestles to crush and mix chemicals. During the Second World War, black agate beads mined from Queensland, Australia were used in the turn and bank indicators of military aircraft.
Agates, particularly moss agates, were first used during the Stone Age to make tools such as arrow and spear points, needles, and hide scrapers. Artifacts from as early as 7000 BCE have been found in Mongolia, and the Natufian people of the Levant are known to have made knives and arrowheads from moss agate as early as 10000 BCE. Agate jewelry from Sumeria has been dated to c. 2500 BCE, and the Ancient Egyptians, Mycenaeans, and Ancient Rome all used agate in their jewelry. Archaeological recovery at the Knossos site on Crete illustrates the role of agates in Bronze Age Minoan culture.C. Michael Hogan. 2007. Knossos fieldnotes, Modern Antiquarian The ornamental use of agate was common in ancient Greece, in assorted jewelry and in the of Greek warriors.
Idar-Oberstein was a historically important location in Germany that made use of agate on an industrial scale, dating back to c. 1375 CE. Originally, locally found agates were used to make all types of objects for the European market, but it became a globalized business around the turn of the 20th century. Idar-Oberstein began to import large quantities of agate from Brazil, as ship's ballast. Making use of a variety of proprietary chemical processes, they produced colored beads that were sold around the globe.
Organic aniline dyes derived from coal tar began to be used later in the 19th century, which allowed for the production of agates of additional colors such as pink and purple. While the colors produced by the classical methods are typically permanent, the colors produced by organic dyes can fade with exposure to light or heat. Organic dyes can also only penetrate a short distance into the agate from the exposed surfaces. The practice of artificially treating agates remains popular today, and dyed Brazilian agates in particular are very common on the global market.
Larger agates are often cut into halves or slices with circular diamond saws. They can then be polished with lapidary grinding, sanding, and polishing wheels of successively greater grit sizes. Smaller agates and crushed agate fragments can alternatively be polished using Tumble finishing or vibratory polishers. This equipment can generate large quantities of silica dust. Respiratory diseases such as silicosis, and a higher incidence of tuberculosis among workers involved in the agate industry, have been studied in India and China.
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