Agate (/ˈæɡɪt/AG-it) is a variously translucent, banded variety ofchalcedony. Agate stones are characterized by alternating bands of different colored chalcedony and may also include visiblequartz 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, bandedchert forms via different processes and is opaque. Agates primarily form as nodules withinvolcanic rock, but they can also form inveins orsilicified fossils. Agate has been popular as agemstone injewelry 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 is composed principally of chalcedony,[1] amicroscopic (microcrystalline) and submicroscopic (cryptocrystalline) form of quartz that grows in fibers. The chemical composition of quartz isSiO2, also known assilica. Normally, between 1% and 20% of the "quartz" in chalcedony is actuallymoganite, a quartzpolymorph.[5] Unlikemacroscopic (macrocrystalline) quartz, which isanhydrous, chalcedony normally contains very small amounts of water bound to its crystal structure.[5][4]: 11
Agate contains multiple layers, or bands, of chalcedony fibers.[1] The fibers are twisted,[6][4]: 11 forming ahelical shape.[7] There are two different types of chalcedony fibers: length-slow (also known asquartzine) 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).[5]
Agate can sometimes contain small amounts ofopal, anamorphous, hydrated form of silica.[4]: 11 Agates also frequently contain macrocrystalline quartz, particularly in the center.[8][4]: 18
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,[8] and agate banding has never been successfully replicated in the lab.[7][9]
Agates are most commonly found asnodules within the cavities of volcanic rocks[10] such asbasalt,andesite, andrhyolite. These cavities, calledvesicles (amygdaloids when filled),[4]: 12 are gas bubbles that were trapped inside the lava when it cooled.[10][8][4]: 12 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.[4]: 12–15 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 fromvolcanic ash ortuff deposits and decomposing plant or animal matter.[4]: 11 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.[4]: 84
Inwall-banded agates, chalcedony fibers grow radially from the vesicle walls inward, perpendicular to the direction of the bands.[1][11] The vesicle walls are often coated with thin layers ofceladonite orchlorite,[8][4]: 18 soft, greenphyllosilicate minerals that form from the reaction of hot, silica-rich water with the rock.[8] This coating provides a rough surface for the chalcedony fibers to form on, initially as radialspherulites. The rough surface also causes agate husks to have a pitted appearance once the coating has been weathered away or removed.[4]: 18–19 Sometimes, the spherulites grow around mineral inclusions, resulting in eyes, tubes, and sagenitic agates.[1]
The first layer of spherulitic chalcedony is typically clear, followed by successive growth bands of chalcedony alternated with chemically precipitated color bands, primarilyiron oxides.[4]: 13 The center is often macrocrystalline quartz,[8] which can also occur in bands and possibly forms when there is not enough chemically bound water in the silica gel to promote chalcedonypolymerization.[4]: 18 When the silica concentration of the gel is too low, a hollow center forms, called an agategeode. 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 invarieties such asamethyst orsmoky quartz.[4]: 17
Level-banded agates form when chalcedony precipitates out of solution in the direction of gravity, resulting in horizontal layers of microscopic chalcedony spherulites.[1] 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.[4]: 19
Enhydro agates, orenhydros, form when liquid water becomes trapped within an agate (or chalcedony) nodule or geode, often long after its formation.[12][13]
Agates can also form within rock fissures, calledveins.[4]: 11–12 Vein agates form in a manner similar to nodular agates (see above),[4]: 13 and they include lace agates such asblue lace agate and crazy lace agate. Veins may form in either volcanic rock orsedimentary rock.[4]: 50
Less commonly, agates can form as nodules within sedimentary rocks such aslimestone,dolomite or tuff. These agates form when silica replaces another mineral, or silica-rich water fills cavities left by decomposed plant or animal matter.[4]: 11–12 Sedimentary agates also includefossil agates, which form when silica replaces the original composition of an organic material.[14] This process is calledsilicification, a form ofpetrification. Examples includepetrified wood,[15] agatized coral,[16] and Turritella agate (Elimia tenera).[17] Although these fossils are often referred to as being "agatized", they are only true agates if they contain bands.[1]
Agates are broadly separated into two categories based on the type of banding they exhibit.[18][4]: 24, 36 Wall banding, also calledconcentric banding oradhesional banding, occurs when agate bands follow the shape of the cavity they formed in.Level banding, also calledwater-level banding,gravitational banding,horizontal banding,parallel banding, orUruguay-type banding, occurs when agate bands form in straight, parallel lines. Level banding is less common and usually occurs together with wall banding.[1]
Agate exhibiting wall banding (top) and level banding (bottom)
Fortification agates are any wall-banded agates with tight, well-defined bands.[4]: 29 They get their name from their appearance which resembles the walls of afort. Fortification agates are one the most common varieties, and they are what most people think of when they hear the word "agate".[19]
Lace agates exhibit alace-like pattern of bands with many swirls, eyes, bends, and zigzags. Unlike most agates, they usually form in veins instead of nodules.[4]: 31
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 withfault 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.[20][4]: 24, 28 They are a form ofbreccia, which is a textural term for any rock composed of angular fragments.[20][21]
Eye agates have one or more circular, concentric rings on their surface.[22] These "eyes" are actuallyhemispheres that form on the husk of the agate and extend inward like a bowl.[23]
Sagenitic agates, orsagenites, haveacicular (needle-shaped) inclusions of another mineral, usuallyanhydrite,aragonite, goethite,rutile, or azeolite. Chalcedony often forms tubes around these crystals and may eventually replace the original mineral, resulting in apseudomorph.[4]: 34 The term "sagenite" was originally a name for a type of rutile, and laterrutilated quartz. It has since been used to describe any quartz variety with acicular inclusions of any mineral.[24]
Tube agates contain tunnel-like structures that extend all the way through the agate.[25] These "tubes" may sometimes be banded or hollow, or both. Tube agates form when chalcedony grew around sagenitic inclusions embedded within the agate, formingstalactitic 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.[4]: 27, 35
Dendritic agates have dark-colored, fern-like patterns (dendrites) that form on the surface or in the spaces between bands.[26][4]: 25 They are composed of manganese or iron oxides.Moss agates exhibit amoss-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 achlorite 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 ofhematite) or bright yellow (from inclusions ofgoethite).[4]: 32–33 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.[1][27]
Iris agates have bands that are fine enough that when thinly sliced, they cause white light to bediffracted into itsspectral colors. This "iris effect" usually occurs in colorless agates, but it can also occur in brightly colored ones.[4]: 30
Brazilian agate with classic fortification banding
Agates with level banding are traditionally calledonyx, although the formal definition of the term onyx refers to color pattern, not the shape of the bands.[28] Accordingly, the nameonyx is also used for wall-banded agates. Onyx is also frequently misused as a name for bandedcalcite. The name originates from the Greek word for the human nail, which has parallel ridges.[4]: 37 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.[29]
Agates are very common, and they have been found on every continent,[4]: 5 including Antarctica.[30] In addition to the structural varieties detailed in the previous section, numerous geological, local, and trade names are used to describe agates from different localities.[4]: 7–9 Below is a table of agate varieties from different regions of the world.
Irregularly-shaped, reddish, banded agate nodules, typically surrounding a hollow cavity lined with macroscopic quartz, but sometimes completely filled
Often large, up to 0.9 m (3.0 ft) in diameter and over 120 kg (260 lb), commonly pale yellow, gray, or colorless (usually sold artificially dyed), are more colorful or contain structural features
Agate is frequently used as a gemstone injewelry such aspins,brooches,necklaces,earrings, andbracelets. Agates have also historically been used in the art ofhardstone carving to make knives,inkstands,seals,marbles, and other objects. Today, they are widely used to make beads, decorative displays, carvings, andcabochons, as well as face-polished and tumble-polished specimens of varying size and origin. Agate collecting is a popular hobby, and agate specimens can be found in numerous gift shops, museums, galleries, and private collections.[4]: 159–168
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 accuratelaboratory balances andmortars and pestles to crush and mix chemicals. During theSecond World War, black agate beads mined fromQueensland, Australia were used in the turn and bank indicators of military aircraft.[4]: 168–169
Agates, particularly moss agates, were first used during theStone 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 theNatufian people of theLevant are known to have made knives and arrowheads from moss agate as early as 10000 BCE. Agate jewelry fromSumeria has been dated to c. 2500 BCE, and theAncient Egyptians,Mycenaeans, andRomans all used agate in their jewelry.[4]: 159–163 Archaeological recovery at theKnossos site onCrete illustrates the role of agates inBronze AgeMinoan culture.[39] The ornamental use of agate was common inancient Greece, in assorted jewelry and in theseal stones of Greek warriors.[40]
Idar-Oberstein was a historically important location in Germany that made use of agate on an industrial scale, dating back to c. 1375 CE.[4]: 52 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.[41]
A 6.8 kg (15 lb) barrel full of tumble-polished agate andjasper
The "Rubens Vase" (Byzantine Empire). Carved in high relief from a single piece of agate, most likely created in an imperial workshop for a Byzantine emperor.
Composite image of an agate slice showing natural color at the top and various artificial colors below
Many pale or dull agates are artificially treated to enhance their colors and make them more appealing to consumers. Chalcedony is one of the earliest stones to be artificially enhanced,[42] with heating having been used for centuries to produce the rich red color of carnelian.[43] Many varieties of chalcedony, including agate, are relatively porous and absorbdyes well.[42][43] The classical methods[44] of staining agates were developed in the early 19th century in Idar-Oberstein, Germany. After the agates were cut and cleaned, they were soaked for several days in a particular inorganic dye or sugar solution depending on the desired color to be achieved. This was often followed by an acid bath and/or heating ("burning") tooxidize the compounds:[42]
Red agates were produced either by burning alone, or if not enough natural iron was present in the stones, by first soaking them in a solution ofiron nitrate and then burning them to formiron oxide.[42]
Green agates were produced using solutions ofnickel orchromiumsalts followed by burning.[42]
Black agates were produced by soaking the stones in a sugar solution and then immersing them insulfuric acid to carbonize the sugars;[42] brown agates can also be produced using a similar method.[43]
Yellow agates can be produced using hydrochloric acid followed by burning.[43]
Organicaniline dyes derived fromcoal tar began to be used later in the 19th century,[42] 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.[44] 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.[4]: 157
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.[4]: 151–155 Smaller agates and crushed agate fragments can alternatively be polished usingrock tumblers or vibratory polishers. This equipment can generate large quantities of silica dust. Respiratory diseases such assilicosis, and a higherincidence oftuberculosis among workers involved in the agate industry, have been studied in India and China.[45][46][47]
^Bates, R. L.; Jackson, J. A. (1987).Glossary of Geological Terms (3rd ed.). Alexandria, Virginia: American Geological Institute. p. 788. Retrieved9 March 2025.
^"Enhydro Agate".mindat.org. Hudson Institute of Mineralogy. Retrieved9 March 2025.
^"Fossil Agate".mindat.org. Hudson Institute of Mineralogy. Retrieved9 March 2025.
^"Petrified Wood".mindat.org. Hudson Institute of Mineralogy. Retrieved9 March 2025.
^"Agatized coral".mindat.org. Hudson Institute of Mineralogy. Retrieved9 March 2025.
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