Agate
19.6 kg (43 lb) specimen of crazy lace agate from Chihuahua, Mexico next to a tennis ball; 38.2 cm (15.0 in) wide
General
Category	Tectosilicate minerals, quartz group, chalcedony variety
Formula	SiO2 (silicon dioxide)
Crystal system	Trigonal (quartz) or monoclinic (moganite)
Identification
Color	Often multicolored; commonly colorless, pale blue to black, red to orange, yellow, white, brown, pink, purple; rarely green
Crystal habit	Cryptocrystalline silica
Cleavage	None
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	6.5–7
Luster	Waxy, vitreous when polished
Streak	White
Diaphaneity	Transparent to opaque (usually translucent)
Specific gravity	2.60–2.64
Density	2.6 g/cm³
Optical properties	Uniaxial (+)
Refractive index	1.530-1.543
Birefringence	Up to 0.004
Pleochroism	Absent
Dispersion	None
References	[1][2][3]

Agate (/ˈæɡɪt/ AG-it) is a banded, fibrous variety of chalcedony. Agate stones are characterized by alternating bands of different colored chalcedony and sometimes include macroscopic quartz. 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 more properly classified solely as varieties of chalcedony. Agates are primarily formed as nodules within volcanic rock, but they can also form in veins or in sedimentary rock. 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 dyed to enhance their color.

Agate was given its name by Theophrastus, a Greek philosopher and naturalist. He discovered the stone c. 350 BCE along the shoreline of the River Achates (Ancient Greek: Ἀχάτης), now the Dirillo River, on the Italian island of Sicily, which at the time was a Greek territory.^{[4]: 52, 162}

Agates are most commonly found as nodules within the cavities of volcanic rocks^[5] such as basalt, andesite, and rhyolite. These cavities, called vesicles (amygdaloids when filled),^[4]: 12 are gas bubbles that were trapped inside the lava when it cooled.^{[5][6][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. 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.^[4]: 11 Agates are much harder than the rocks they form in, so they are frequently found detached from their host rock.

Geologists generally understand the early stages of agate formation, but the specific processes that result in band development are still widely debated. Since they form in cavities within host rock, agate formation cannot be directly observed,^[6] and unlike most other crystals, agates have never been successfully lab-grown.^[7]

Agate is composed of multiple bands, or layers, of chalcedony fibers,^[8] specifically length-fast chalcedony fibers and sometimes quartzine (length-slow chalcedony fibers).^[1] Agate can also contain opal, an amorphous, hydrated form of silica.^[4]: 11 In wall-banded agates, the fibers grow radially from the vesicle walls inward, perpendicular to the direction of the bands.^[1][9] The vesicle walls are often coated with thin layers of celadonite or chlorite,^[6][4]: 18 soft, green phyllosilicate minerals that form from the reaction of hot, silica-rich water with the rock.^[6] 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.^{[4]: 18–19} 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.^[4]: 13 The center is often macrocrystalline quartz (quartz with visible crystals),^[6] which can also occur in bands and forms when there is not enough water in the silica gel to promote chalcedony polymerization.^[4]: 18 When the silica concentration of the gel is too low, a hollow center forms, called an agate geode.^{[10][4]: 17} Quartz forms crystals around the cavity, with the apex of each crystal pointing towards the center. Occasionally, the quartz may be colored, 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.^[1] Enhydro agates, or enhydros, form when water becomes trapped within an agate (or chalcedony) nodule or geode, often long after its formation.^[11][12]

Agates can also form within rock fissures, called veins.^{[4]: 11–12} Vein agates form in a manner similar to nodular agates, and they include most lace agates, such as blue lace agate and crazy lace agate.

Less commonly, agates can form as nodules within sedimentary rock, 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.^[4]: 11-12 Sedimentary agates also include fossil agates, which form when silica replaces the original composition of an organic material.^[13] This process is called silicification, a form of petrification. Examples include petrified wood,^[14] agatized coral,^[15] and Turritella agate, fossilized Elimia tenera. Although these fossils are often referred to as being "agatized," they are only true agates when they are banded.^[1]

Agates are broadly separated into two categories based the type of banding they exhibit.^{[6][4]: 24, 36} 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.^[1]

Fortification agates have very tight, well-defined bands. They get their name from their appearance resembling 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."^[6]

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.^[4]: 31

Faulted agates occur when agate bands are 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 have also had their bands broken apart and re-cemented with chalcedony, but they consist of disjointed band fragments at random angles.^{[6][4]: 24, 28} They are a form of breccia, which is a textural term for any rock composed of angular fragments.^[6][16]

Eye agates have one or more circular, concentric rings on their surface.^[17] These "eyes" are actually hemispheres that form on the husk of the agate and extend inward like a bowl. Tube agates contain tunnel-like structures that extend all the way through the agate. These "tubes" may sometimes be banded or hollow, or both. Both tube and eye agates form when chalcedony grows around a needle-shaped crystal of another mineral 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.^{[6][4]: 27, 35}

Dendritic agates have dark-colored, fern-like patterns (dendrites) on the surface or the spaces between bands.^[6][4]: 25 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 (from inclusions of hematite) or bright yellow (from inclusions of goethite).^{[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][18]

Iris agates have bands that are so microscopically fine 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.^[4]: 30

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.^[4]: 34 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.^[19]

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.^[20] 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.^[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.^[21]

Thunder eggs are frequently level-banded, however they may also have wall banding. Level banding is also common in Lake Superior agates.^[6]

• 
  Agate with both wall banding (top) and level banding (bottom)
• 
  Brazilian agate with classic fortification banding
• 
  Crazy lace agate
• 
  Tumbled Lake Superior eye agates
• 
  Dendritic agate from India
• 
  Moss agate cabochons
• 
  Iris agate from petrified wood
• 
  Level-banded agate
• 
  Onyx agate
• 
  Level-banded thunder egg from Oregon, USA

Agates are very common, and they have been found on every continent, including Antarctica. In addition to names used to describe their structure, numerous geological, local, and trade names are applied to agates from different localities.^[4]: 7–9 Below is a list of known agate localities and the names of the agates that are found there. This list is not exhaustive.

• Blue lace agate is a pale blue and white lace agate found primarily in Namibia. These agates formed in dolomite associated with igneous rock.
• Botswana agates are found in basaltic rocks of the Permian age in Botswana. They feature contrasting bands of purple, pink, black, grey, and white. Like Lake Superior agates, they are typically small, averaging 2.5-5 cm (1-2 in) in diameter.
• Malawi agates are typically bright red or orange with contrasting white bands, but some are pink and blue. They can be found in Malawi, and they likely formed in volcanic rock of Permian age.
• Agates have also been found in Egypt, Madagascar, South Africa, and Zimbabwe.^{[4]: 131–137}

• 
  Blue lace agate
• 
  Botswana agate
• 
  Malawi agate

• White and clear banded agates have been found by scientists at Bellingshausen Station, a Russian outpost on King George Island.^[22]

• India has produced agates since as early as the 11th century. These include carnelian agates, moss agates, and dendritic agates.
• Yemen is home to a variety of agate called mocha stone, named after the port city of Mocha (also spelled Mokha or Mukha) on the Red Sea. These agates likely formed in tuff deposits of Late Oligocene and Early Miocene age.
• Agates have also been found in Iran, Mongolia, China,^{[4]: 138–143} and Russia.^[1]

• Queensland agates, found in the State of Queensland, often occur in colors that are rarely found in agates from other regions, such as green and yellow-green. They formed in basaltic lava flows of the Late Permian period. Level banding is common in Queensland agates, while inclusions are uncommon. Queensland is also home to several kinds of thunder egg, which are thought to date from the Early Cretaceous period.
• Agates have also been found in Tasmania and other regions of Australia.^{[4]: 144–149}

• Agate was discovered in Sicily by the Greek scholar Theophrastus in 350 BC. At the time, Sicily was a colony of ancient Greece. The name "agate" comes from the Achates River, the Greek name for what is now known as the Dirillo River. Agates in Sicily formed in balsaltic lavas and pyroclastic rocks of the Pilocene epoch.
• Germany is a well-known historic source of agate. Agates mined from volcanic rock of the Permian period have been processed in Idar-Oberstein since at least 1375, but possibly as early as the Roman Empire. Agates from the Idar-Oberstein area are often red and pink, but other colors have also been observed. Many museum specimens include features such as eyes, tubes, moss, plumes, and sagenite.
• Scotland is an abundant source of a wide variety of agates. There are at least 50 main agate localities in Scotland. Scottish agates have been popular in jewelry for several hundred years, particularly during the Victorian era. They formed in two types of rock: andesite from the Early Devonian period and basalt from the Tertiary period. The andesite deposits are more significant and extend from Stonehaven in the northeast to just south of Ayr in the southwest. The basaltic agates are confined to the islands off the west coast of Scotland and are collectively called the Small Isles agates. The colors of Scottish agates vary, and bands may be different shades of blue, grey, purple, pink, brown, orange, or red.
• Pot stones or potato stones are irregular agate nodules or geodes found in Bristol and Somerset, England. They typically consist of a reddish, banded agate surrounding a hollow cavity lined with macroscopic quartz, although some are completely filled with agate. Other varieties of agate have also been found elsewhere in England.
• Agates can also be found in Wales, the Czech Republic, Poland, France,^{[4]: 52–75} and many other European countries.^[1]

• 
  German agate from Idar-Oberstein
• 
  Close-up of a Scottish agate from Ayrshire
• 
  Potato stone from England
• 
  Agate from Czech Republic

• Coldwater agates, such as the Lake Michigan cloud agate, did not form under volcanic processes, but instead formed within the limestone and dolomite strata of marine origin. Like volcanic-origin agates, Coldwater agates formed from silica gels that lined pockets and seams within the bedrock. These agates are typically less colorful, with banded lines of grey and white chalcedony.^[23]
• Crazy lace agate is a brightly colored lace agate from Mexico with a complex pattern, demonstrating randomized distribution of contour lines and circular droplets, scattered throughout the rock. The stone is typically colored red and white but is also seen to exhibit yellow and grey combinations as well.^[24] Crazy lace agate is a vein agate that formed in sedimentary rock of the late Cretaceous period.^[4]: 121
• Dugway geodes are a type of thunder egg found in Utah. They are typically light grey to blue and often contain hollow cavities lined with drusy quartz.^[4]: 92
• Fairburn agates are rare fortification agates named for Fairburn, South Dakota. They are sedimentary agates that originated during the Pennsylvanian period, and then weathered from their host rock and redeposited during the Oligocene epoch in parts of South Dakota and Nebraska.^[25]
• Laguna agate is a brightly colored agate variety that was first discovered in Ojo Laguna, Chihuahua, Mexico.^[26] It features vibrant bands in shades of red, orange, pink, or purple. Laguna agates formed in andesite and are geologically young. They frequently contain inclusions and many exhibit parallax or shadow banding.^{[4]: 114–115}
• Lake Superior agates are believed to be the world's oldest agates;^[27] they formed as nodules in basalt up to 1.2 billion years ago during the Late Precambrian. These agates are primarily found near the shores of Lake Superior in the U.S. states of Minnesota, Michigan, and Wisconsin, and in the Canadian province of Ontario. They are not named after the lake, but rather the Lake Superior Till, the Pleistocene glacial deposit in which they are found.^{[4]: 83–84} This deposit also extends into portions of Iowa, Nebraska, Kansas, and Missouri, and Lake Superior agates have been carried south by the Mississippi River into Arkansas and Louisiana. Lake Superior agates have bands in shades of red, orange, yellow, brown, white, and grey. They can contain a variety of structural features, including eyes, tubes, sagenite, dendrites, faults, and geodes.^[6]
• Lysite agate is a vein agate named after Lysite Mountain, Wyoming. It is frequently colorful and may contain moss and plumes in addition to bands.^[4]: 79
• Nebraska blue agate is a sedimentary agate with dendritic patterns that formed during the Oligocene epoch. It can be found throughout northwestern Nebraska and southwestern South Dakota.^[4]: 80
• Oregon is known for several different varieties of agate. It is probably best known for its thunder eggs, which form in rhyolitic ash and have a brown rhyolite shell that is usually filled with blue and white agate.^[4]: 99 Holley blue agate (also spelled "Holly blue agate") is a rare lavender to blue agate found only near Holley, Oregon.^[4]: 103
• Patuxent River stone is a red and yellow form of agate only found in Maryland, where it is the state gem.^[28]
• Sweetwater agates are small moss agates found in Miocene age sandstone near Sweetwater River, Wyoming. They also contain brown or black dendrites and fluoresce under UV light.^[4]: 79
• Turritella agate is a brown fossil agate formed from the remains of an extinct species of freshwater snail (Elimia tenera) with an elongated spiral shell. The name is a misnomer; it was originally thought to be the fossil of a different genus of gastropods, Turritella. It is found in the Green River Formation of Wyoming.^[29]
• Other varieties of agate have also been found in nearly every U.S. state, northern Mexico, and in the Canadian provinces of Nova Scotia, Manitoba, and British Columbia.^{[4]: 76–121}

• 
  Crazy lace agate
• 
  Dugway geode from Utah
• 
  Fairburn agate from western South Dakota
• 
  Laguna agate
• 
  Rough Lake Superior agate from Keweenaw Peninsula, Michigan
• 
  Thunder egg from Oregon
• 
  Holley blue agate from Oregon
• 
  Turritella agate (Elimia tenera)
• 
  Agate from British Columbia

• Brazilian agate is probably one of the largest agates. They can reach 0.9 m (3 ft) in diameter and weigh over 120 kg (300 lb). Brazilian agate is found primarily as nodules and geodes in decomposed volcanic ash and basalt of Late Permian age. The largest deposits are in the Brazilian state of Rio Grande do Sul, but significant amounts can also be found throughout southeastern Brazil. Some specimens can be very colorful and include features such as eyes, tubes, dendrites, and sagentite. However, most Brazilian agate that is mined is naturally pale yellow, gray, or colorless and artificially dyed before being brought to market.
• Condor agates are found in the Mendoza province of Argentina. They typically have bright red and yellow fortification banding and may contain mossy or sagenitic inclusions. Other varieties of agate can also be found in the Patagonia area of Argentina, including crater agate (typically hollow nodules with black and red bands) and puma agate (agatized coral).
• Uruguay was the first major source of agates in South America. Agates were discovered there in 1830, but sources in neighboring Brazil became more popular in the late 19th and 20th centuries.
• Agates have also been found in Chile and Peru.^{[4]: 122–131}

• 
  Natural Brazilian agate
• 
  Dyed Brazilian agate
• 
  Condor agate
• 
  Agate from Uruguay

Agate is popular as a gemstone in jewellery such as pins, brooches, necklaces, and bracelets. Agates are also used in the decorative arts to make knives, inkstands, marbles, beads, seals, decorative displays, cabochons, carvings and Intarsia art as well as face-polished and tumble-polished specimens of varying size and origin. Industrial uses of agate exploit its hardness, ability to retain a highly polished surface finish and resistance to chemical attack. It has traditionally been used to make knife-edge bearings for laboratory balances and precision pendulums, and sometimes to make mortars and pestles to crush and mix chemicals.

Agate is one of the most common materials used in the art of hardstone carving, and has been recovered at a number of ancient sites, indicating its widespread use in the ancient world; for example, archaeological recovery at the Knossos site on Crete illustrates its role in Bronze Age Minoan culture.^[30] The ornamental use of agate was common in ancient Greece, in assorted jewelry and in the seal stones of Greek warriors.^[31] Bead necklaces with pierced and polished agate date back to the 3rd millennium BCE in the Indus Valley civilisation. It has also been used for centuries for leather burnishing tools. Idar-Oberstein was historically a center which made use of agate on an industrial scale. 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.^[32]

• 
  A 6.8 kg (15 lb) barrel full of tumble-polished agate and jasper
• 
  Gold Roman signet ring with portrait of emperor Commodus in niccolo agate, 180-200 CE, found in Tongeren, Gallo-Roman Museum (Tongeren)
• 
  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.
• 
  Victorian banded agate earrings
• 
  Maryland Agate - Cut and illuminated to show internal structures

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.^[33][34][35]

• List of minerals – List of minerals with Wikipedia articles

• "Agates", School of Natural Resources, University of Nebraska-Lincoln (retrieved 27 December 2014).