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
Formula (repeating unit)	SiO2 (silicon dioxide)
Crystal system	Trigonal or monoclinic
Identification
Color	Banded
Crystal habit	Cryptocrystalline silica
Cleavage	None
Fracture	Conchoidal, with very sharp edges
Mohs scale hardness	6.5–7
Luster	Waxy
Streak	White
Diaphaneity	Translucent
Specific gravity	2.58–2.64
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, which comes in a wide variety of colors. Agates are primarily formed within volcanic rock, but can also form in sedimentary rock. The ornamental use of agate was common in ancient Greece, in assorted jewelry and in the seal stones of Greek warriors, while bead necklaces with pierced and polished agate date back to the 3rd millennium BCE in the Indus Valley civilisation.

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 are formed from the gases trapped within the liquid volcanic material forming vesicles. Cavities 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.

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

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, 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 circular banded spots 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 with the agate. They are often bright red (from inclusions of hematite) or bright yellow (from inclusions of goethite).

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.

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

Types (by variety name)

Blue lace agate is found in Africa and is especially hard. Crazy lace agate, typically found in Mexico, is often brightly colored with a complex pattern, demonstrating randomized distribution of contour lines and circular droplets, scattered throughout the rock. The stone is typically coloured red and white but is also seen to exhibit yellow and grey combinations as well.

Turritella agate (Elimia tenera) is formed from the shells of fossilized freshwater Turritella gastropods with elongated spiral shells. Similarly, coral, petrified wood, porous rocks and other organic remains can also form agate.

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.

Greek agate is a name given to pale white to tan colored agate found in the former Greek colony of Sicily as early as 400 BCE. The Greeks used it for making jewelry and beads.

Brazilian agate is found as sizable geodes of layered nodules. These occur in brownish tones inter-layered with white and gray. It is often dyed in various colors for ornamental purposes.

Polyhedroid agate forms in a flat-sided shape similar to a polyhedron. When sliced, it often shows a characteristic layering of concentric polygons. It has been suggested that growth is not crystallographically controlled but is due to the filling-in of spaces between pre-existing crystals which have since dissolved.

Other forms of agate include Holley blue agate (also spelled "Holly blue agate"), a rare dark blue ribbon agate found only near Holley, Oregon; Lake Superior agate; Carnelian agate (has reddish hues); Botswana agate; condor agate; Binghamite, a variety found only on the Cuyuna iron range (near Crosby) in Crow Wing County, Minnesota; fire agate showing an iridescent, internal flash or "fire", the result of a layer of clear agate over a layer of hydrothermally deposited hematite; and Patuxent River stone, a red and yellow form of agate only found in Maryland.

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  Agatized coral
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  "Turritella agate" (Elimia tenera)
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  Crazy lace agate
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  Botzwana agate

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. 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).