''This article is about the semi-precious stone. For other uses, see [[Agate (disambiguation)]].''

{{Infobox mineral
| name = Agate
| category = Quartz variety
| boxwidth =
| boxbgcolor =
| image = mossagate.pebble.750pix.jpg
| imagesize =
| caption = Moss agate pebble, 2.5 cm (1 inch) long
| formula = Silica, SiO₂
| molweight =
| color = White to grey, light blue, orange to red, black.
| habit = [[Cryptocrystalline]] silica
| system = [[Rhombohedral Microcrystalline]]
| twinning =
| cleavage = None
| fracture = [[Conchoidal]] with very sharp edges.
| mohs = 7
| luster = Waxy
| refractive = 1.530-1.540
| opticalprop =
| birefringence = up to +0.004 (B-G)
| pleochroism = Absent
| streak = White
| gravity = 2.58-2.64
| density =
| melt =
| fusibility =
| diagnostic =
| solubility =
| diaphaneity =
| other =
}}

'''Agate''' is a microcrystalline variety of quartz ([[silicon dioxide|silica]]), chiefly [[chalcedony]], characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks but can be common in certain [[metamorphic rocks]].<ref name="wilder">{{cite book
| first= David D. Alt
| last=Donald W. Hyndman
| authorlink=
| coauthors=
| year= 2002
| title= Roadside Geology of Oregon
| edition=18th
| publisher= Mountain Press Publishing Company
| location= Missoula, Montana
| pages = p. 286
| id= ISBN 0-87842-063-0 }}</ref>

Colorful agates and other [[chalcedony|chalcedonies]] were obtained over 3,000 years ago from the Achates River, now called [[Dirillo]], in [[Sicily]].<ref>{{cite web
|url=http://www.gem.org.au/agate.htm
|title= Agate Creek Agate
|accessdate=2007-07-01
}}</ref>

The stone was given its name by [[Theophrastus]], a Greek philosopher and naturalist, who discovered the stone along the shore line of the river Achates sometime between the 4th and 3rd centuries BC.

== Formation and characteristics ==
Most agates occur as [[Nodule (geology)|nodules]] in [[volcanic]] rocks or ancient [[lava]]s where they represent cavities originally produced by the disengagement of [[volatiles]] in the molten mass which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. Such agates, when cut transversely, exhibit a succession of parallel lines, often of extreme tenuity, giving a banded appearance to the section. Such stones are known as banded agate, riband agate and striped agate.

In the formation of an ordinary agate, it is probable that waters containing [[silica]] in solution -- derived, perhaps, from the decomposition of some of the silicates in the lava itself -- percolated through the rock and deposited a siliceous coating on the interior of the vapour-vesicles. Variations in the character of the solution or in the conditions of deposition may cause a corresponding variation in the successive layers, so that bands of chalcedony often alternate with layers of crystalline [[quartz]]. Several vapour-vesicles may unite while the rock is still [[Viscosity|viscous]], and thus form a large cavity which may become the home of an agate of exceptional size; thus a [[Brazil]]ian [[geode]] lined with [[amethyst]] and weighing 67 tons was exhibited at the [[Dusseldorf Exhibition]] of 1902. Perhaps the most comprehensive review of agate chemistry is a recent text by Moxon cited below.

The first deposit on the wall of a cavity, forming the "skin" of the agate, is generally a dark greenish mineral substance, like [[celadonite]], [[delessite]] or "[[green earth]]", which are rich in [[iron]] probably derived from the decomposition of the [[augite]] in the enclosing volcanic rock. This green silicate may give rise by alteration to a brown [[iron oxide]] ([[limonite]]), producing a rusty appearance on the outside of the agate-nodule. The outer surface of an agate, freed from its matrix, is often pitted and rough, apparently in consequence of the removal of the original coating. The first layer spread over the wall of the cavity has been called the "priming", and upon this base zeolitic minerals may be deposited.

[[Image:Agate banded 750pix.jpg|thumb|left|250px|Banded agate (agate-like [[onyx]]). The specimen is 2.5 cm (1 inch) wide.]]

Many agates are hollow, since deposition has not proceeded far enough to fill the cavity, and in such cases the last deposit commonly consists of quartz, often amethyst, having the apices of the [[crystal]]s directed towards the free space so as to form a crystal-lined cavity, or geode.

On the disintegration of the matrix in which the agates are embedded, they are set free. The agates are extremely resistant to weathering and remain as nodules in the [[soil]] or are deposited as [[gravel]] in streams and shorelines.

== Types of agate ==
{{refimprove|date=July 2007}}
[[Image:Agatized Coral.jpg|thumb|Agatized Coral]]
A [[Mexico|Mexican]] agate, showing only a single eye, has received the name of "[[cyclops (rock)|cyclops]] agate." Included matter of a green, golden, red, black or other color or combinations embedded in the chalcedony and disposed in filaments and other forms suggestive of vegetable growth, gives rise to dendritic or [[moss agate]] (named varieties include Maury Mountain, Marston Ranch, Sheep Creek and others). '''Dendritic''' agates have beautiful fern like patterns on them formed due to the presence of manganese and iron oxides. Other types of included matter deposited during agate-building include sagenitic growths (radial mineral crystals) and chunks of entrapped detritus (such as sand, ash, or mud). Occasionally agate fills a void left by decomposed vegetative material such as a tree limb or root and is called limb cast agate due to its appearance.

Turritella agate is formed from [[fossil]] Turritella shells silicified in a chalcedony base. Turritella are spiral marine [[gastropod]]s having elongated, spiral shells composed of many whorls. Similarly, [[coral]], [[petrified wood]] and other organic remains or porous rocks can also become agatized. Agatized coral is often referred to as [[Petoskey stone|Petoskey]] agate or stone.
[[Image:Montana Moss Agate.jpg|thumb|Montana Moss Agate]]

California's "Mojave Blue" agate has gained a great deal of attention in the past several years. This pastel blue or blue-gray agate cuts into attractive cabochons for jewelry and, in the hands of an expert carver, makes outstanding carvings.

Greek agate is a name given to pale white to tan colored agate found in Sicily back to 400 B.C. The Greeks used it for making jewelry and beads. Today any agate of this color from Sicily, once an ancient Greek colony, is called Greek agate. Yet the stone had been around centuries before that and was known to both the Sumerians and the Egyptians, who used the gem for decoration and religious ceremony.

Another type of agate is Brazilian agate, which is found as sizable geodes of layered nodules. These occur in brownish tones interlayered with white and gray. Quartz forms within these nodules, creating a striking specimen when cut opposite the layered growth axis. It is often dyed in various colors for ornamental purposes.

Certain stones, when examined in thin sections by transmitted light, show a diffraction spectrum due to the extreme delicacy of the successive bands, whence they are termed [[rainbow agates]]. Often agate coexists with layers or masses of opal, jasper or crystalline quartz due to ambient variations during the formation process.

Other forms of agate include carnelian agate (usually exhibiting reddish hues), Botswana agate, [[Ellensburg blue agate]], blue lace agate, plume agates (such as Carey, Graveyard Point, Sage, St. Johns, Teeter Ranch and others), tube agate (with visible flow channels), fortification agate (which exhibit little or no layered structure), fire agate (which seems to glow internally like an opal) and Mexican crazy-lace agate (which exhibits an often brightly colored, complex banded pattern).

== Agate beliefs ==
[[Image:Agatebots.jpg|thumb|right|Faceted Botswana agate]]
In many traditions agate is believed to cure the stings of [[scorpion]]s and the bites of [[snake]]s, soothe the mind, prevent [[Disease|contagion]], still [[thunder]] and [[lightning]], promote [[eloquence]], secure the favour of the powerful, and bring victory over enemies.{{Fact|date=April 2007}} Persian [[magi]] are also known to have prized agate rings in their work and beliefs.

Some followers of Pagan religions also believe agate is a crystal whose powers can be used for love, mental clarity, and good luck in card games.

[[Shia]] Muslims often wear an agate ring on the right hand, the stone engraved with the name of [[Allah]], [[Ali]], or the names of the other eleven [[Imam]]s. It is known as "aqaq" or "agag" in Persian.

== Uses in industry ==
Industry uses agates chiefly to make ornaments such as pins, brooches, paper knives, inkstands, marbles and seals. Because of its hardness and ability to resist acids, agate is used to make mortars and pestles to crush and mix chemicals. Because of the high polish possible with agate it has been used for centuries for leather burnishing tools.

==See also==
*[[List of minerals]]
*[[Lake Superior agate]]
*[[Fire Agate]]
==Notes==
<references/>
==References==
{{commonscat}}
* [http://www.minsocam.org/MSA/collectors_corner/arc/silicanom.htm ''The Nomenclature of Silica'' by Gilbert Hart, American Mineralogist, Volume 12, pages 383-395, 1927]
*[http://www.gemstone.org/gem-by-gem/english/agate.html International Colored Gemstone Association]
*[http://www.mindat.org/min-51.html Mindat data]
*Schumann, Walter. ''Gemstones of the World''. 3rd edition. New York: Sterling, 2006.
*http://www.gemstone.org/gem-by-gem/english/agate.html
*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.
*Cross, Brad L. and Zeitner, June Culp. "Geodes. Nature's Treasures". Bardwin Park, California, Gem Guides Book Co. 2005.

{{Gems & Crystals}}
[[Category:Minerals]]
[[Category:Quartz varieties]]

[[ar:عقيق]]
[[bg:Ахат]]
[[cs:Achát]]
[[da:Agat]]
[[de:Achat]]
[[et:Ahhaat]]
[[es:Ágata (mineral)]]
[[eo:Agato]]
[[fa:عقیق]]
[[fr:Agate]]
[[gl:Ágata]]
[[is:Agat]]
[[it:Agata (minerale)]]
[[he:שבו]]
[[lv:Ahāts]]
[[lt:Agatas]]
[[hu:Achát]]
[[nl:Agaat]]
[[ja:メノウ]]
[[pl:Agat]]
[[pt:Ágata]]
[[ro:Agat]]
[[ru:Агат]]
[[sk:Achát]]
[[sl:Ahat]]
[[sr:Ахат]]
[[fi:Akaatti]]
[[sv:Agat]]
[[th:โมรา]]
[[tr:Akik]]
[[uk:Агат (мінерал)]]
[[zh:瑪瑙]]