Agate

For other uses, see Agate (disambiguation).

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	Chalcedony variety, quartz group, tectosilicate
Formula (repeating unit)	SiO2 (silicon dioxide)
Crystal system	Trigonal (quartz) or monoclinic (moganite)
Identification
Color	Often multicolored; commonly colorless, pale blue to black, red to orange, white, brown, pink, purple; rarely yellow, green
Crystal habit	Cryptocrystalline silica
Cleavage	None
Fracture	Conchoidal, with very sharp edges
Mohs scale hardness	6.5–7
Luster	Waxy, vitreous when polished
Streak	White
Diaphaneity	Translucent
Specific gravity	2.58–2.64
Density	2.6 g/cm³
Refractive index	1.530–1.540
Birefringence	Up to +0.004 (B-G)
Pleochroism	Absent

Agate (/ˈæɡɪt/ AG-it) is a fibrous, banded 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.

Etymology

The stone was given its name by Theophrastus, a Greek philosopher and naturalist, who discovered the stone along the shore line of the Dirillo River or Achates (Ancient Greek: Ἀχάτης) in Sicily, sometime between the 4th and 3rd centuries BCE.

Formation and properties

Hollow agate

Agates are most commonly found as nodules within the cavities of volcanic rocks. These cavities, called vesicles, are formed from the gases trapped within the liquid volcanic material. The vesicles are then filled in with silica-rich fluids from the volcanic material. Layers are then deposited on the walls of the cavity slowly working their way inwards. The first layer deposited on the cavity walls is commonly known as the priming layer. Variations in the character of the solution or in the conditions of deposition may cause a corresponding variation in the successive layers. These variations in layers result in bands of chalcedony, often alternating with layers of crystalline quartz forming banded agate. Hollow agates can also form due to the deposition of liquid-rich silica not penetrating deep enough to fill the cavity completely. Agate will form crystals within the reduced cavity, and the apex of each crystal may point towards the center of the cavity.

The priming layer is often dark green, but can be modified by iron oxide resulting in a rust like appearance. Agate is very durable and is often found detached from its host matrix, which may have eroded more quickly. Once removed, the outer surface is usually pitted and rough from filling the cavity of its former matrix. Agates have also been found in sedimentary rocks, normally in limestone, dolomite or ash tuff; these sedimentary rocks acquire cavities often from decomposed branches or other buried organic material. If silica-rich fluids are able to penetrate into these cavities agates can be formed . These types of agate are commonly called limb casts.

Types (by structure)

Agates are broadly separated into two categories based 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 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."

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 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. 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. 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.

Dendritic agates have dark-colored, fern-like patterns (dendrites) on the surface or 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 (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 agates according to the mineralogical definition.

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.

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.

Enhydro agates form when tiny inclusions of water become trapped within an agate.

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 be 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.

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

Varieties (by locality)

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. Below is a list of known agate localities and the names of the agates that are found there. This list is not exhaustive.

Africa

• 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.

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  Blue lace agate
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  Botswana agate
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  Malawi agate

Antarctica

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

Agate from King George Island, Antarctica

Asia

• 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, and Russia.

Rough agates from the Gobi Desert in Mongolia

Australia

• 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.

Queensland agate with level banding

Europe

• 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, and many other European countries.

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

North America

• 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.
• 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. Crazy lace agate is a vein agate that formed in sedimentary rock of the late Cretaceous period.
• 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.
• 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.
• Laguna agate is a brightly colored agate variety that was first discovered in Ojo Laguna, Chihuahua, Mexico. 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.
• Lake Superior agates are believed to be the world's oldest agates; 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. 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.
• 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.
• 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.
• 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. Holley blue agate (also spelled "Holly blue agate") is a rare lavender to blue agate found only near Holley, Oregon.
• Patuxent River stone is a red and yellow form of agate only found in Maryland, where it is the state gem.
• 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.
• 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.
• Other varieties of agate have also been found in nearly every U.S. state, and in the Canadian provinces of Nova Scotia, Manitoba, and British Columbia.

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

South America

• 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.

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  Natural Brazilian agate
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  Dyed Brazilian agate
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  Condor agate
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  Agate from Uruguay

Uses

Agate drinking horn, Tang dynasty

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. The ornamental use of agate was common in ancient Greece, in assorted jewelry and in the seal stones of Greek warriors. 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.

The decorative arts use it to make ornaments such as pins, brooches or other types of jewellery, paper knives, inkstands, marbles and seals. Agate is also still used today for decorative displays, cabochons, beads, carvings and Intarsia art as well as face-polished and tumble-polished specimens of varying size and origin. Idar-Oberstein was one of the centers which made use of agate on an industrial scale. Where in the beginning locally found agates were used to make all types of objects for the European market, this became a globalized business around the turn of the 20th century: Idar-Oberstein imported 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. Agates have long been used in arts and crafts. The sanctuary of a Presbyterian church in Yachats, Oregon, has six windows with panes made of agates collected from the local beaches.

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.

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

Health impact

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.

See also

• Amber
• Amethyst
• Aqeeq
• Aquamarine
• Citrine
• Diamond
• Emerald
• Garnet
• Geode
• Kyanite
• Labradorite
• List of minerals – List of minerals with Wikipedia articles
• Lithophysa
• Moonstone
• Opal
• Peridot
• Rose Quartz
• Swiss Blue Topaz
• Thunderegg
• Tiger's Eye
• Topaz
• Tourmaline
• Turquoise

General and cited references

• Cross, Brad L. and Zeitner, June Culp. Geodes: Nature's Treasures. Bardwin Park, Calif.: Gem Guides Book Co. 2005.
• Hart, Gilbert "The Nomenclature of Silica", American Mineralogist, Volume 12, pages 383–395, 1927
• International Colored Gemstone Association, "Agate: banded beauty"
• "Agate", Mindat.org, Hudson Institute of Mineralogy
• Moxon, Terry. Agate: Microstructure and Possible Origin. Doncaster, S. Yorks, UK: Terra Publications, 1996.
• Pabian, Roger, et al. Agates: Treasures of the Earth. Buffalo, New York: Firefly Books, 2006.
• Schumann, Walter. Gemstones of the World. 3rd edition. New York: Sterling, 2006.

External links

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