Agate

Agate /ˈæɡət/ is a common rock formation, consisting of chalcedony and quartz as its primary components,[1] consisting of a wide variety of colors. Agates are primarily formed within volcanic and metamorphic rocks. Decorative uses of agates are known to date back to Ancient Greece and are used most commonly as decorations or jewelry.

For other uses, see Agate (disambiguation).

Agate
19.6 kg (43.2 lb) Specimen of "Crazy Lace" agate from Chihuahua, Mexico; the specimen is 38.2 cm (15.0 in) wide
General
Category	Chalcedony variety
Formula (repeating unit)	SiO2 silicon dioxide
Crystal system	Rhombohedral (microcrystalline)
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

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 Dirillo River or Achates (Ancient Greek: Ἀχάτης) in Sicily,[2] sometime between the 4th and 3rd centuries BC.[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

Agate minerals have the tendency to form on or within pre-existing rocks, creating difficulties in accurately determining their time of formation.[5] Their host rocks have been dated to have formed as early as the Archean Eon. 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.[6] Cavities are then filled in with silica-rich fluids from the volcanic[6] material, layers are deposited on the walls of the cavity slowly working their way inwards.[7] The first layer deposited on the cavity walls is commonly known as the priming layer.[8] 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 alternate with layers of crystalline quartz forming banded agate.[6] Hollow agates can also form due to the deposition of liquid-rich silica not penetrating deep enough to fill the cavity completely.[9] Agate will form crystals within the reduced cavity, the apex of each crystal may point towards the center of the cavity.

The priming layer are often dark green, but can be modified by iron oxide resulting in a rust like appearance.[8] Agate is a very durable and therefore is often seen detached from its eroding matrix, 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,[6] normally in limestone or dolomite; these sedimentary rocks require 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.[6]

Hollow Agate

Types

Lace agate is a variety that exhibits a lace-like pattern with forms such as eyes, swirls, bands or zigzags. Blue lace agate is found in Africa and is especially hard.[10] 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.[11]

Moss agate, as the name suggests, exhibits a moss like pattern and is of a greenish colour. The coloration is not created by any vegetative growth, but rather through the mixture of chalcedony and oxidized iron hornblende. Dendritic agate also displays vegetative features, including fern-like patterns formed due to the presence of manganese and iron oxides.[12]

Turritella agate (Elimia tenera) is formed from the shells of fossilized freshwater Turritellas, gastropods with elongated spiral shells. Similarly, coral, petrified wood, porous rocks and other organic remains can also form agate.[13]

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.[14]

Greek agate is a name given to pale white to tan colored agate found in Sicily, once a Greek colony, back to^{[clarification needed]} 400 BC. 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. 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.

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 only found near Holley, Oregon; Lake Superior agate; Carnelian agate (has reddish hues); Botswana agate; Plume agate; Condor agate; Tube agate containing visible flow channels or pinhole-sized "tubes"; Fortification agate with contrasting concentric banding reminiscent of defensive ditches and walls around ancient forts; 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; Patuxent River stone, a red and yellow form of agate only found in Maryland; Enhydro agate contains tiny inclusions of water, sometimes with air bubbles.

• 
  Agatized coral
• 
  Dendritic agate
• 
  "Turritella agate" (Elimia tenera)
• 
  Crazy lace agate

Uses in industry and art

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. 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.[15] 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.[16]

• 
  A 6.8 kg (15 lb) barrel full of tumble-polished agate and jasper
• 
  The Holy Grail of Valencia, the cup made from agate carved during the time of Christ[17]
• 
  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 ear rings

Health impact

Respiratory diseases such as silicosis and higher incidence of tuberculosis among workers involved in the agate industry have been reported from India and China.[18][19][20]

See also

• Topaz
• Opal
• List of minerals – A list of minerals for which there are articles on Wikipedia

Notes

1. Wang, Yifeng; Merino, Enrique (1990-06-01). "Self-organizational origin of agates: Banding, fiber twisting, composition, and dynamic crystallization model". Geochimica et Cosmochimica Acta. 54 (6): 1627–1638. Bibcode:1990GeCoA..54.1627W. doi:10.1016/0016-7037(90)90396-3. ISSN 0016-7037.
2. "Agate Creek Agate". Archived from the original on 16 July 2007. Retrieved 2007-07-01.
3. "Achates". Retrieved 19 September 2014.
4. C. Michael Hogan. 2007. Knossos fieldnotes, Modern Antiquarian
5. "Agate: Mineral information, data and localities". www.mindat.org. Retrieved 2020-02-11.
6. T, Moxon (2006). "Agate and chalcedony from igneous and sedimentary hosts aged from 13 to 3480 Ma: a cathodoluminescence study". Mineralogical Magazine. 70 (5): 485–498. Bibcode:2006MinM...70..485M. doi:10.1180/0026461067050347. Archived from the original on 2006. Retrieved October 1, 2006.
7. Walger, Eckart; Mattheß, Georg; von Seckendorff, Volker; Liebau, Friedrich (August 2009). "The formation of agate structures: models for silica transport, agate layer accretion, and for flow patterns and flow regimes in infiltration channels". www.ingentaconnect.com. Retrieved March 3, 2020.
8. "Metaphysical Properties of Yellow Skin Agate - Stone Treasures". Stone Treasures by the Lake. Retrieved 2020-02-27.
9. "Agate chalcedony: The mineral Agate information and pictures". www.minerals.net. Retrieved 2020-02-27.
10. Simmons, Robert; Ahsian, Naisha (2007). The Book of Stones: Who They Are and What They Teach. North Atlantic Books. ISBN 978-1-55643-668-0.
11. Atkinson, Bill; Ackerman, Diane (2004). Within the Stone: Photography. BrownTrout Publishers. ISBN 978-0-7631-8189-5.
12. Schumann, Walter (2009). Gemstones of the World. Sterling Publishing Company, Inc. ISBN 978-1-4027-6829-3.
13. Grant, Ember (2016-06-08). The Second Book of Crystal Spells: More Magical Uses for Stones, Crystals, Minerals... and Even Salt. Llewellyn Worldwide. ISBN 978-0-7387-4844-3.
14. Garvin, Paul (2010-09-13). Iowa's Minerals: Their Occurrence, Origins, Industries, and Lore. University of Iowa Press. ISBN 978-1-60938-014-4.
15. Background Article on Idar Oberstein
16. "Agate Windows - Community Presbyterian Church". Archived from the original on 1 March 2012. Retrieved 19 September 2014.
17. Beltrán, Antonio (1984). El Santo Cáliz de la Catedral de Valencia (in Spanish) (2 ed.). Valencia: Nacher.
18. Chaudhury, Nayanjeet; Phatak, Ajay; Paliwal, Rajiv (January 2012). "Co-morbidities among silicotics at Shakarpur: A follow up study". Lung India. 29 (1): 6–10. doi:10.4103/0970-2113.92348. PMC 3276038. PMID 22345906.
19. Jiang, CQ; Xiao, LW; Lam, TH; Xie, NW; Zhu, CQ (July 2001). "Accelerated silicosis in workers exposed to agate dust in Guangzhou, China". American Journal of Industrial Medicine. 40 (1): 87–91. doi:10.1002/ajim.1074. PMID 11439400.
20. Tiwari, RR; Narain, R; Sharma, YK; Kumar, S (September 2010). "Comparison of respiratory morbidity between present and ex-workers of quartz crushing units: Healthy workers' effect". Indian Journal of Occupational and Environmental Medicine. 14 (3): 87–90. doi:10.4103/0019-5278.75695. PMC 3062020. PMID 21461160.

References

• Cross, Brad L. and Zeitner, June Culp. Geodes: Nature's Treasures. Bardwin Park, California, 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).