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how to find agates (agate identification)
how to find agates (agate identification)
Published: 2014/12/06
Channel: How to Alaska
Agate Expo, Cedarburg, WI, July 2016
Agate Expo, Cedarburg, WI, July 2016
Published: 2016/07/10
Channel: Cathy Odell
Lake Superior Agates 2017 "part 2" KILLER AGATE FOUND!
Lake Superior Agates 2017 "part 2" KILLER AGATE FOUND!
Published: 2017/03/03
Channel: Michael Roy
CUTTING AN AGATE OPEN!
CUTTING AN AGATE OPEN!
Published: 2016/07/09
Channel: JOJOBEAN GEOLOGY
Steven Universe (Clip) The Crystal Gems Vs. Holy Blue Agate - That Well Be All.
Steven Universe (Clip) The Crystal Gems Vs. Holy Blue Agate - That Well Be All.
Published: 2017/01/03
Channel: Lapis Lazuli
LAZINTERVIEW - AGATE (Game-Dev Terbesar Indonesia)  , Nasib Industri Game Indonesia, DLL
LAZINTERVIEW - AGATE (Game-Dev Terbesar Indonesia) , Nasib Industri Game Indonesia, DLL
Published: 2017/12/11
Channel: The Lazy Monday
Where do agates come from?
Where do agates come from?
Published: 2015/01/27
Channel: KeyWestBluesX
Discovering - The Agate Lady
Discovering - The Agate Lady
Published: 2017/06/06
Channel: 906 Outdoors
Astrix & Ritmo - Agate
Astrix & Ritmo - Agate
Published: 2016/02/29
Channel: Astrix Official
How To Identify Agates
How To Identify Agates
Published: 2016/03/09
Channel: MudCat TV
Civilizing an Agate
Civilizing an Agate
Published: 2013/02/26
Channel: gotrocksinhead
Moss Agate - The Crystal of the Wildness
Moss Agate - The Crystal of the Wildness
Published: 2017/06/29
Channel: Adam Barralet
Agate
Agate
Published: 2016/03/03
Channel: Anmol Gupta
Rockhounding - Carnilien Agate and Petrified Wood HUNT
Rockhounding - Carnilien Agate and Petrified Wood HUNT
Published: 2017/05/22
Channel: Todd Armstrong
DIY Cheap Agate Coasters//Concrete Coasters/Gold Leaf Dipped Bowl
DIY Cheap Agate Coasters//Concrete Coasters/Gold Leaf Dipped Bowl
Published: 2017/03/06
Channel: Cesar Gonzalez
Metaphysical Properties of Agate Stones
Metaphysical Properties of Agate Stones
Published: 2012/09/24
Channel: Crystal Guy
Ellensburg Blue Agates: Downtown Geology Lecture Series
Ellensburg Blue Agates: Downtown Geology Lecture Series
Published: 2013/08/15
Channel: Central Washington University
Finding Agates on the Beach
Finding Agates on the Beach
Published: 2016/06/01
Channel: Tim Blair
Madagascar Agate Collection Fun
Madagascar Agate Collection Fun
Published: 2017/08/31
Channel: AgateMaster
Fecund Creek, pt 2: The Agate
Fecund Creek, pt 2: The Agate
Published: 2017/02/24
Channel: gotrocksinhead
DIY Faux Agate Coasters TUMBLR Inspired
DIY Faux Agate Coasters TUMBLR Inspired
Published: 2014/10/13
Channel: HelloMaphie
Hunting for Kentucky agate
Hunting for Kentucky agate
Published: 2015/04/22
Channel: Kentucky.com
BD-305 Agate Cutter
BD-305 Agate Cutter
Published: 2015/11/12
Channel: Barranca Diamond
2017 Agate Hunt
2017 Agate Hunt
Published: 2017/10/21
Channel: Mncat30
Lake Superior Agate Diving
Lake Superior Agate Diving
Published: 2016/10/23
Channel: Chris Cooper
Agate Garden Update, Oregon Rockhound 2016
Agate Garden Update, Oregon Rockhound 2016
Published: 2016/11/18
Channel: Brian Fletcher
Agate from Agate Creek
Agate from Agate Creek
Published: 2016/06/22
Channel: Russell Elliot
#SconnieAgateHunter - Lake Superior Agate Hunting Tips and Tricks
#SconnieAgateHunter - Lake Superior Agate Hunting Tips and Tricks
Published: 2017/03/08
Channel: HuntourageTV
Xenoblade Chronicles 2 Agate
Xenoblade Chronicles 2 Agate's Awakening
Published: 2017/12/05
Channel: QP GAMES
Blue Lace Agate - The Crystal of Gentleness
Blue Lace Agate - The Crystal of Gentleness
Published: 2016/08/22
Channel: Adam Barralet
Let
Let's Make an Agate | SpeedPaint
Published: 2017/07/05
Channel: Twist3dDisast3r
Steven Universo - 10 Curiosidades Sobre A Holly Blue Agate
Steven Universo - 10 Curiosidades Sobre A Holly Blue Agate
Published: 2017/02/28
Channel: Hora Cartoon
MÉTÉO AGATE DU 09 12 2017
MÉTÉO AGATE DU 09 12 2017
Published: 2017/12/08
Channel: agate59
SLICING AGATE & JASPER FOR ROCK TUMBLING
SLICING AGATE & JASPER FOR ROCK TUMBLING
Published: 2017/08/28
Channel: tess99991
Chancing 1000 Agate Amulets
Chancing 1000 Agate Amulets
Published: 2017/09/16
Channel: Slipperyjim8
Healing with Blue Lace Agate
Healing with Blue Lace Agate
Published: 2014/08/04
Channel: Crystal Healing
Agate carving master in NE China
Agate carving master in NE China
Published: 2014/12/02
Channel: New China TV
AGATE GEODE Prospecting | Liz Kreate
AGATE GEODE Prospecting | Liz Kreate
Published: 2015/11/03
Channel: Liz Kreate
Steven Universe - Everytime Holly Blue Agate does something b*tchy
Steven Universe - Everytime Holly Blue Agate does something b*tchy
Published: 2017/01/05
Channel: Jaekoff
Agate hunting and rock collecting PART 1
Agate hunting and rock collecting PART 1
Published: 2016/07/31
Channel: FOUND IT!
Most recent trip to san jacinto river to hunt agate jasper and petrified wood
Most recent trip to san jacinto river to hunt agate jasper and petrified wood
Published: 2016/06/29
Channel: nathan gandara
Agate Hunt: Winter Carnelian of the PNW
Agate Hunt: Winter Carnelian of the PNW
Published: 2017/02/05
Channel: Sourcerer Stones
Ryszard Krukowski Real Time Fire Agate Carving Tutorial
Ryszard Krukowski Real Time Fire Agate Carving Tutorial
Published: 2017/02/19
Channel: Mark Anderson
Crystal Healing Agate Stone
Crystal Healing Agate Stone
Published: 2012/10/01
Channel: Sean Leifer
Agate & Rock Hunting at Ocean Shores, WA- directions & tips on where to find agates on Damon Point
Agate & Rock Hunting at Ocean Shores, WA- directions & tips on where to find agates on Damon Point
Published: 2017/08/01
Channel: Tim Blair
Supernanny UK The Agate family 001
Supernanny UK The Agate family 001
Published: 2015/04/20
Channel: عروب المسفر
Carnelian Agate: Find It & Finish It
Carnelian Agate: Find It & Finish It
Published: 2013/09/04
Channel: Dave Smith
Fire Agate - The Crystal of the Supporting Flame
Fire Agate - The Crystal of the Supporting Flame
Published: 2017/12/06
Channel: Adam Barralet
Digging Fire Agate in California
Digging Fire Agate in California
Published: 2017/12/11
Channel: Mining America
My Biggest Agate So Far
My Biggest Agate So Far
Published: 2013/04/04
Channel: Mncat30
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WIKIPEDIA ARTICLE

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Agate
Agate banded 750pix.jpg
Banded agate (agate-like onyx); the specimen is 2.5 cm (0.98 in) wide
General
Category Chalcedony variety
Formula
(repeating unit)
SiO2 silicon dioxide
Crystal system Rhombohedral Microcrystalline
Identification
Color brown 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 /ˈæɡət/ is a rock consisting primarily of cryptocrystalline silica, chiefly chalcedony, alternating with microgranular quartz. It is characterized by its fineness of grain and variety of color. Although agates may be found in various kinds of host rock, they are classically associated with volcanic rocks and can be common in certain metamorphic rocks.[1]

Etymology and history[edit]

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) in present-day Sicily,[2] sometime between the 4th and 3rd centuries BCE.[3] Colorful agates and other chalcedonies were obtained over 3,000 years ago from the Achates River, now called Dirillo.

Ancient use[edit]

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[edit]

Botswana agate

Most agates occur as nodules in volcanic rocks or ancient lavas, in former cavities produced by volatiles in the original 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 gel in the interior of the 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.

When the matrix in which the agates are embedded disintegrates, 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 along shorelines.

Types of agate[edit]

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. Enhydro agate contains tiny inclusions of water, sometimes with air bubbles.

Turritella agate is formed from silicified fossil Elimia tenera (erroneously considered Turritella) shells. E. tenera are spiral freshwater 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.[citation needed]

Greek agate is a name given to pale white to tan colored agate found in Sicily back to 400 BC. 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.

Brazilian agate 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.

Lace agate is a variety that exhibits a lace-like pattern with forms such as eyes, swirls, bands or zigzags (if these predominate, it is called lattice agate). Crazy lace agate, found in Mexico, is often brightly colored and complexly patterned. Blue lace agate is found in Africa and is especially hard.[5]

Polyhedroid agate is agate which has grown in a flat-sided shape similar to a polyhedron.[6] When sliced, it often shows a characteristic layering of concentric polygons. Polyhedroid agate is thought to be found only in Paraíba State, Brazil. 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.[7]

Other forms of agate include Holley blue agate (also spelled "Holly blue agate"), a rare dark blue ribbon agate only found near Holley, Oregon;[8] Lake Superior agate; carnelian agate (has reddish hues); Botswana agate; plume agate; condor agate, tube agate (with 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 in Montana; fire agate (showing 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, where it is the state gem.

Uses in industry and art[edit]

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.[9] 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.[10]

Health impact[edit]

Respiratory diseases such as silicosis and higher incidence of tuberculosis among workers involved in the agate industry has been reported from India and China.[12][13][14]

See also[edit]

Notes[edit]

  1. ^ Donald W. Hyndman, David D. Alt (2002). Roadside Geology of Oregon (18th ed.). Missoula, Montana: Mountain Press Publishing Company. p. 286. ISBN 0-87842-063-0. 
  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. ^ Polk, Patti (2016). Collecting Rocks, Gems and Minerals: Identification, Values and Lapidary Uses. F+W Media, Inc. p. 80. ISBN 9781440246159. 
  6. ^ Plodowski, G., and Werner, R.; Achate: Kleine Senckenberg-Reihe, No. 12 (1981), p. 34, fig. 20
  7. ^ O'Donoghue, M.; Quartz, Butterworth-Heinemann (1987), p. 82. ISBN 978-0408014625
  8. ^ "Socio-Cultural Impacts of Water Resource Development in the Santiam River Basin" (PDF). Oregon State University: Water Resources Research Institute. October 1970. p. 9. Retrieved February 19, 2012. 
  9. ^ Background Article on Idar Oberstein
  10. ^ "Agate Windows - Community Presbyterian Church". Retrieved 19 September 2014. 
  11. ^ Beltrán, Antonio (1984). El Santo Cáliz de la Catedral de Valencia (in Spanish) (2 ed.). Valencia: Nacher. 
  12. ^ 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 3276038Freely accessible. PMID 22345906. 
  13. ^ 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. 
  14. ^ 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 3062020Freely accessible. PMID 21461160. 

References[edit]

External links[edit]

  • Media related to Agate at Wikimedia Commons
  • "Agates", School of Natural Resources, University of Nebraska-Lincoln (retrieved 27 December 2014).

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