Agate

Source: Wikipedia, the free encyclopedia.
For other uses, see Agate (disambiguation).
Agate
Specific gravity
2.58–2.64
Refractive index1.530–1.540
Birefringenceup to +0.004 (B-G)
PleochroismAbsent

Agate (

Indus Valley civilisation
.

Etymology

The stone was given its name by

Ancient Greek: Ἀχάτης) in Sicily,[3] sometime between the 4th and 3rd centuries BCE.[4]

Formation and properties

Botswana agate
Hollow agate

Agate minerals have the tendency to form on or within pre-existing rocks, creating difficulties in accurately determining their time of formation.

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 alternating 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, and the apex of each crystal may point towards the center of the cavity.

The priming layer is often dark green, but can be modified by iron oxide resulting in a rust like appearance.[8] Agate is very durable, and is often found detached from its host matrix, which has eroded away. 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 acquire 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]

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 (

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 the former Greek colony of Sicily as early as 400 BCE. 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. 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.

Iris agate is a finely-banded and usually colorless agate, that when thinly sliced, exhibits spectral decomposition of white light into its constituent colors, requiring 400 to up to 30,000 bands per inch.[15]

Other forms of agate include Holley blue agate (also spelled "Holly blue agate"), a rare dark blue ribbon agate found only 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; and enhydro agate, which contains tiny inclusions of water, sometimes with air bubbles.

Agate is a versatile gemstone that is often used in jewelry making. Agate is favored for its durability, with a Mohs scale hardness rating of 6.5–7. It is known for its colorful, banded patterns and wide range of hues. Agate is found in a wide range of colors, including shades of red, orange, yellow, green, blue, purple, and pink, as well as black and white. Agate is generally more affordable than other gemstones.


  • A piece of translucent pink agatized coral, with a "ruffled" appearance along the top edge
    Agatized coral
  • Dendritic agate
    Dendritic agate
  • 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)
  • Crazy lace agate
    Crazy lace agate

Uses

Agate drinking horn, Tang dynasty

Agate is one of the most common materials used in the art of

leather burnishing
tools.

The decorative arts use it to make ornaments such as

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.[17] 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.[18]

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.

  • A 6.8 kg (15 lb) barrel full of tumble-polished agate and jasper
    A 6.8 kg (15 lb) barrel full of tumble-polished agate and jasper
  • Gold Roman signet ring with portrait of emperor Commodus in niccolo agate, 180-200 CE, found in Tongeren, Gallo-Roman Museum (Tongeren)
    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.
    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 earrings
    Victorian banded agate earrings
  • Maryland Agate - Cut and illuminated to show internal structures
    Maryland Agate - Cut and illuminated to show internal structures

Health impact

Respiratory diseases such as

incidence of tuberculosis among workers involved in the agate industry, have been studied in India and China.[19][20][21]

See also

Citations

  1. ISSN 0016-7037
    .
  2. ^ "Masterpiece of Greek Art Found in the Griffin Warrior Tomb". Smithsonian. Smithsonian Institution. 7 November 2017.
  3. ^ "Agate Creek Agate". Archived from the original on 16 July 2007. Retrieved 2007-07-01.
  4. ^ "Achates". Archived from the original on 26 August 2023. Retrieved 19 September 2014.
  5. ^ "Agate: Mineral information, data and localities". www.mindat.org. Archived from the original on 2020-02-15. Retrieved 2020-02-11.
  6. ^
    S2CID 54607138. Archived
    from the original on March 13, 2022. 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. Archived from the original on June 4, 2018. Retrieved March 3, 2020.
  8. ^ a b "Metaphysical Properties of Yellow Skin Agate - Stone Treasures". Stone Treasures by the Lake. 24 March 2019. Archived from the original on 2020-02-26. Retrieved 2020-02-27.
  9. ^ "Agate chalcedony: The mineral Agate information and pictures". www.minerals.net. Archived from the original on 2020-03-16. Retrieved 2020-02-27.
  10. ISBN 978-1-55643-668-0. Archived
    from the original on 2023-08-26. Retrieved 2020-10-29.
  11. ISBN 978-0-7631-8189-5. Archived
    from the original on 2023-08-26. Retrieved 2020-10-29.
  12. ISBN 978-1-4027-6829-3. Archived
    from the original on 2023-08-26. Retrieved 2020-10-29.
  13. ISBN 978-0-7387-4844-3. Archived
    from the original on 2023-08-26. Retrieved 2020-10-29.
  14. ISBN 978-1-60938-014-4. Archived
    from the original on 2023-08-26. Retrieved 2020-10-29.
  15. ^ "Iris Agate". Archived from the original on 2022-07-01. Retrieved 2022-05-27.
  16. ^ C. Michael Hogan. 2007. Knossos fieldnotes, Modern Antiquarian Archived 2018-07-11 at the Wayback Machine
  17. ^ "Background Article on Idar Oberstein". Archived from the original on 2008-12-23. Retrieved 2008-11-27.
  18. ^ "Agate Windows - Community Presbyterian Church". Archived from the original on 1 March 2012. Retrieved 19 September 2014.
  19. PMID 22345906
    .
  20. PMID 11439400
    .
  21. PMID 21461160
    .

General and cited references

External links

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