Play Video
1
Civilizing an Agate
Civilizing an Agate
::2013/02/26::
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2
Carnelian Agate: Find It & Finish It
Carnelian Agate: Find It & Finish It
::2013/09/04::
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3
Agate Albekeite (SAPŅU DZIESMIŅA)
Agate Albekeite (SAPŅU DZIESMIŅA)
::2011/04/18::
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4
Digging for Agates - Agate Creek Queensland, Australia (re-uploaded)
Digging for Agates - Agate Creek Queensland, Australia (re-uploaded)
::2014/03/04::
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5
Cutting Haida Gwaii Agate
Cutting Haida Gwaii Agate
::2012/12/18::
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6
The World of FIRE AGATE - episode 1
The World of FIRE AGATE - episode 1
::2012/06/09::
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7
DIY Faux Agate Coasters TUMBLR Inspired
DIY Faux Agate Coasters TUMBLR Inspired
::2014/10/12::
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8
Agate Hunter
Agate Hunter
::2011/05/10::
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9
Agate Albekeite (ŅAM, ŅAM)
Agate Albekeite (ŅAM, ŅAM)
::2011/04/04::
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10
My Biggest Agate So Far
My Biggest Agate So Far
::2013/04/03::
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11
Gemstone cutting & Polishing Agate
Gemstone cutting & Polishing Agate
::2012/06/01::
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12
Metaphysical Properties of Agate Stones
Metaphysical Properties of Agate Stones
::2012/09/24::
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13
Agate Albekeite (GOLDEN EYE)
Agate Albekeite (GOLDEN EYE)
::2011/03/14::
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14
modeling Nail Gel Agate, liquid silver, setting forms
modeling Nail Gel Agate, liquid silver, setting forms
::2014/05/30::
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15
How Was it Made? An Agate Teapot by Michelle Erickson
How Was it Made? An Agate Teapot by Michelle Erickson
::2013/07/17::
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16
Agate Albekeite (PUTEKLIS)
Agate Albekeite (PUTEKLIS)
::2011/03/07::
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17
Faux (Fake) Agate Pendant Polymer Clay Tutorial / Arcilla Polimérica
Faux (Fake) Agate Pendant Polymer Clay Tutorial / Arcilla Polimérica
::2014/09/04::
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18
Agate Albekeite "Nokavētais reiss"
Agate Albekeite "Nokavētais reiss"
::2014/07/12::
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19
Agate Village - Pokémon XD: Gale of Darkness Music Extended
Agate Village - Pokémon XD: Gale of Darkness Music Extended
::2013/08/11::
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20
Kathy Mattea | Agate Hill
Kathy Mattea | Agate Hill
::2014/05/22::
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21
Flint Knapping Agate With Traditional Tools and Techniques Part 1
Flint Knapping Agate With Traditional Tools and Techniques Part 1
::2010/07/08::
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22
Unexpected Lake Superior Agate Hunting
Unexpected Lake Superior Agate Hunting
::2014/08/25::
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23
Supernanny UK The Agate family
Supernanny UK The Agate family
::2014/05/11::
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24
Agate Hunting
Agate Hunting
::2013/11/10::
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Agate hunting on Oregon Santiam River
Agate hunting on Oregon Santiam River
::2012/09/24::
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canari agate  chante
canari agate chante
::2012/05/09::
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Satinette Agate Red Mosaic Canary
Satinette Agate Red Mosaic Canary
::2013/01/10::
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Porsche Macan S Morocco - Footage Agate Grey Metallic
Porsche Macan S Morocco - Footage Agate Grey Metallic
::2014/04/11::
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29
How to Drill a Hole In an Agate Slab.mp4
How to Drill a Hole In an Agate Slab.mp4
::2012/06/20::
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kenari Agate  Red Mozaic Ring FNP 2013 Portugal
kenari Agate Red Mozaic Ring FNP 2013 Portugal
::2013/12/16::
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How to make an agate marble or stone sphere - Part 1
How to make an agate marble or stone sphere - Part 1
::2011/08/03::
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32
agate mosaic
agate mosaic
::2014/02/01::
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33
Fun Way About Agate Studio
Fun Way About Agate Studio
::2012/02/08::
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34
Agate Head
Agate Head
::2009/07/18::
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35
Astrix & Ritmo - Agate
Astrix & Ritmo - Agate
::2014/12/13::
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36
Cut That Agate - Episode 1 - Mojave Lace
Cut That Agate - Episode 1 - Mojave Lace
::2013/11/24::
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37
2013 Porsche 911 Carrera S 991 Agate Grey Espresso Aerokit Cup Sport Design Beverly Hills
2013 Porsche 911 Carrera S 991 Agate Grey Espresso Aerokit Cup Sport Design Beverly Hills
::2013/04/10::
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38
Die besten "Agate Bauer" Songs
Die besten "Agate Bauer" Songs
::2013/12/15::
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39
Agate Hunting on the Oregon Coast
Agate Hunting on the Oregon Coast
::2013/02/08::
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40
The Healing Properties of Moss Agate - A Stone of the Plant Kingdom (April 15, 2012)
The Healing Properties of Moss Agate - A Stone of the Plant Kingdom (April 15, 2012)
::2012/04/15::
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41
Agate Albekeite - DIG A LITTLE DEEPER
Agate Albekeite - DIG A LITTLE DEEPER
::2011/05/09::
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42
Variation Fairy of Generosity (Fairy, scatter bread crumbs) Agate Zalite
Variation Fairy of Generosity (Fairy, scatter bread crumbs) Agate Zalite
::2014/04/12::
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43
Moss Agate   Lets Talk Stones
Moss Agate Lets Talk Stones
::2013/08/06::
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44
The Magic of Crystals: Moss Agate
The Magic of Crystals: Moss Agate
::2013/09/09::
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45
agate bauer
agate bauer
::2008/05/30::
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46
Crystal Healing Agate Stone
Crystal Healing Agate Stone
::2012/09/30::
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47
Metaphysical Properties of Moss Agate
Metaphysical Properties of Moss Agate
::2012/09/25::
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48
Agate Studio Recruitment Video
Agate Studio Recruitment Video
::2012/02/28::
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49
Pokemon Colosseum- Agate Village
Pokemon Colosseum- Agate Village
::2008/11/25::
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50
Kathy Mattea - "Agate Hill"
Kathy Mattea - "Agate Hill"
::2012/10/23::
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From Wikipedia, the free encyclopedia
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For other uses, see Agate (disambiguation).
Agate
Agate banded 750pix.jpg
Banded agate (agate-like onyx); the specimen is 2.5 cm (1 inch) wide
General
Category Quartz variety
Formula
(repeating unit)
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 cryptocrystalline 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[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) 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[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 coating on 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 piece of translucent pink agatized coral, with a "ruffled" appearance along the top edge
Agatized Coral

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.

An irregular dark stone with a flat polished front; many white fragments of elongated, spiral, "corkscrew" shells seem to float in the dark stone
"Turritella agate" (Elimia tenera) from Green River Formation, Wyoming

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.

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.

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 (has reddish hues); Botswana agate; blue lace 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); fire agate (showing internal flash or "fire", the result of a layer of clear agate over a layer of hydrothermally deposited hematite); and 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[edit]

A 15 pound (6.8 kg) tumbler barrel full of glistening tumble-polished agate and jasper.
The "Rubens Vase" (Byzantine Empire). Carved in high relief from a single piece of agate, this extraordinary vase was most likely created in an imperial workshop for a Byzantine emperor.

Industry uses agates chiefly to make ornaments such as pins, brooches or other types of jewelry, 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]

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.[7][8][9]

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. ^ "Achates". Retrieved 19 September 2014. 
  3. ^ "Agate Creek Agate". Archived from the original on 16 July 2007. Retrieved 2007-07-01. 
  4. ^ C. Michael Hogan. 2007. Knossos fieldnotes, Modern Antiquarian
  5. ^ Background Article on Idar Oberstein
  6. ^ "Agate Windows - Community Presbyterian Church". Retrieved 19 September 2014. 
  7. ^ Chaudhury, Nayanjeet; Phatak, Ajay; Paliwal, Rajiv. "Co-morbidities among silicotics at Shakarpur: A follow up study". Lung India 29 (1): 6. doi:10.4103/0970-2113.92348. 
  8. ^ Jiang, CQ; Xiao, LW; Lam, TH; Xie, NW; Zhu, CQ (Jul 2001). "Accelerated silicosis in workers exposed to agate dust in Guangzhou, China.". American journal of industrial medicine 40 (1): 87–91. PMID 11439400. 
  9. ^ Tiwari, RR; Narain, R; Sharma, YK; Kumar, S (Sep 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. PMID 21461160. 

References[edit]

External links[edit]

  • Media related to Agate at Wikimedia Commons
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