agate

Agate
Banded agate (agate-like onyx); the specimen is 2.5 cm (1 inch) wide
General
Category	Quartz variety
Chemical formula	SiO2 silicon dioxide
Identification
Color	White to grey, light blue, orange to red, black. banded
Crystal habit	Cryptocrystalline silica
Crystal system	Rhombohedral Microcrystalline
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 ( /ˈæɡət/) is a microcrystalline variety of 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 and can be common in certain metamorphic rocks.^[1]

Etymology and history

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 (Greek: Ἀχάτης) sometime between the 4th and 3rd centuries BC.^[2] Colorful agates and other chalcedonies were obtained over 3,000 years ago from the Achates River, now called Dirillo, in Sicily.^[3]

Ancient use

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.^[4]

Formation and characteristics

Botswana agate

Most agates occur as nodules in volcanic rocks or ancient lavas 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. Agate has also been known to fill veins or cracks in volcanic or altered rock underlain by granitic intrusive masses. 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 viscous, and thus form a large cavity which may become the home of an agate of exceptional size; thus a Brazilian geode lined with amethyst and weighing 35 tons was exhibited at the Düsseldorf 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.

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 drusy quartz, sometimes amethystine, having the apices of the crystals 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

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Agatized Coral

"Turritella agate" (Elimia tenera) from Green River Formation, Wyoming

A Mexican agate, showing only a single eye, has received the name of 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. Dendritic agates have fern like patterns in 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 silicified fossil Elimia tenera (erroneously considered Turritella) shells. E. tenera are spiral marine gastropods 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 or agate.

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. Even though the stone had been around centuries and was known to both the Sumerians and the Egyptians, both who used the gem for decoration and for playing important parts in their religious ceremonies, any agate of this color from Sicily, once an ancient Greek colony, is called Greek agate.

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 Lake Superior agate, carnelian agate (exhibiting reddish hues), Botswana agate, blue lace agate, plume agates, moss agate, tube agate (with visible flow channels or pinhole-sized 'tubes'), fortification agate (which exhibit little or no banding structure), fire agate (which has internal flash or 'fire', the result of a layer of clear agate over a layer of hydrothermally-deposited hematite), Mexican crazy-lace agate, which often exhibits a brightly colored, complexly banded pattern (also called Rodeo Agate and Rosetta Stone depending on who owned the mine at the time).

Uses in industry and art

Industry uses agates chiefly to make ornaments such as pins, brooches, 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. 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. 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.^[5] 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.^[6]

A 15-pound tumbler barrel full of glistening tumble-polished agate and jasper.

See also

• List of minerals
• Lake Superior agate

Notes

1. ^ Donald W. Hyndman, David D. Alt (2002). Roadside Geology of Oregon (18th ed.). Missoula, Montana: Mountain Press Publishing Company. pp. 286. ISBN 0-87842-063-0. 
2. ^ Achates, Henry George Liddell, Robert Scott, A Greek-English Lexicon, at Perseus
3. ^ "Agate Creek Agate". http://www.gem.org.au/agate.htm. Retrieved 2007-07-01. 
4. ^ C. Michael Hogan. 2007. Knossos fieldnotes, Modern Antiquarian
5. ^ Background Article on Idar Oberstein
6. ^ http://www.yachatspresbyterian.org/webapp/GetPage?pid=211

References

• The Nomenclature of Silica by Gilbert Hart, American Mineralogist, Volume 12, pages 383-395, 1927
• International Colored Gemstone Association
• Mindat data
• Schumann, Walter. Gemstones of the World. 3rd edition. New York: Sterling, 2006.
• 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.

External links

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