Agate

Last updated
Agate
Mexican Crazy Lace Agate - World's Best.jpg
19.6 kg (43 lb) specimen of "Crazy Lace" agate from Chihuahua, Mexico next to a tennis ball; 38.2 cm (15.0 in) wide
General
Category Chalcedony variety
Formula
(repeating unit)
SiO2 (silicon dioxide)
Crystal system Trigonal or monoclinic
Identification
Colorbanded
Crystal habit Cryptocrystalline silica
Cleavage None
Fracture Conchoidal, with very sharp edges
Mohs scale hardness6.5–7
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 ( /ˈæɡɪt/ AG-it) is the banded variety of chalcedony, [1] which comes in a wide variety of colors. Agates are primarily formed within volcanic and metamorphic rocks. The ornamental use of agate was common in Ancient Greece, in assorted jewelry and in the seal stones of Greek warriors, [2] while bead necklaces with pierced and polished agate date back to the 3rd millennium BCE in the Indus Valley civilisation.

Contents

Etymology

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, [3] sometime between the 4th and 3rd centuries BCE. [4]

Formation and properties

Botswana agate Faceted Botswana agate.jpg
Botswana agate
Hollow agate Agate- & quartz-lined geode 5 (32375570960).jpg
Hollow 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 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 ( Elimia tenera) is formed from the shells of fossilized freshwater Turritella 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.

Uses

Agate drinking horn, Tang dynasty Tang -Ma Nao Shou Shou Bei .jpg
Agate drinking horn, Tang dynasty

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. [16] 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. [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.

Health impact

Respiratory diseases such as silicosis, and a higher incidence of tuberculosis among workers involved in the agate industry, have been studied in India and China. [19] [20] [21]

See also

Citations

  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. "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. 1 2 3 4 5 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. 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. 1 2 "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. 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. Archived from the original on 2023-08-26. Retrieved 2020-10-29.
  11. Atkinson, Bill; Ackerman, Diane (2004). Within the Stone: Photography. BrownTrout Publishers. ISBN   978-0-7631-8189-5. Archived from the original on 2023-08-26. Retrieved 2020-10-29.
  12. Schumann, Walter (2009). Gemstones of the World. Sterling Publishing Company, Inc. ISBN   978-1-4027-6829-3. Archived from the original on 2023-08-26. Retrieved 2020-10-29.
  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. Archived from the original on 2023-08-26. Retrieved 2020-10-29.
  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. 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. 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.
  20. 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.
  21. 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.

General and cited references

Related Research Articles

<span class="mw-page-title-main">Amethyst</span> Mineral, quartz variety

Amethyst is a violet variety of quartz. The name comes from the Koine Greek αμέθυστος amethystos from α-a-, "not" and μεθύσκωmethysko / μεθώmetho, "intoxicate", a reference to the belief that the stone protected its owner from drunkenness. Ancient Greeks wore amethyst and carved drinking vessels from it in the belief that it would prevent intoxication.

<span class="mw-page-title-main">Quartz</span> Mineral made of silicon and oxygen

Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is, therefore, classified structurally as a framework silicate mineral and compositionally as an oxide mineral. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.

<span class="mw-page-title-main">Chalcedony</span> Microcrystalline varieties of silica

Chalcedony ( kal-SED-ə-nee, or KAL-sə-doh-nee) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (silicon dioxide).

<span class="mw-page-title-main">Carnelian</span> Yellow-red chalcedony variety

Carnelian is a brownish-red mineral commonly used as a semiprecious stone. Similar to carnelian is sard, which is generally harder and darker; the difference is not rigidly defined, and the two names are often used interchangeably. Both carnelian and sard are varieties of the silica mineral chalcedony colored by impurities of iron oxide. The color can vary greatly, ranging from pale orange to an intense almost-black coloration. Significant localities include Yanacodo (Peru); Ratnapura ; and Thailand. It has been found in Indonesia, Brazil, India, Russia (Siberia), and Germany. In the United States, the official State Gem of Maryland is also a variety of carnelian called Patuxent River stone.

<span class="mw-page-title-main">Celestine (mineral)</span> Sulfate mineral

Celestine (the IMA-accepted name) or celestite is a mineral consisting of strontium sulfate (SrSO4). The mineral is named for its occasional delicate blue color. Celestine and the carbonate mineral strontianite are the principal sources of the element strontium, commonly used in fireworks and in various metal alloys.

<span class="mw-page-title-main">Jasper</span> Chalcedony variety colored by iron oxide

Jasper, an aggregate of microgranular quartz and/or cryptocrystalline chalcedony and other mineral phases, is an opaque, impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. The common red color is due to iron(III) inclusions. Jasper breaks with a smooth surface and is used for ornamentation or as a gemstone. It can be highly polished and is used for items such as vases, seals, and snuff boxes. The density of jasper is typically 2.5 to 2.9 g/cm3. Jaspillite is a banded-iron-formation rock that often has distinctive bands of jasper.

<span class="mw-page-title-main">Chert</span> Hard, fine-grained sedimentary rock composed of cryptocrystalline silica

Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.

<span class="mw-page-title-main">Onyx</span> Banded variety

Onyx is the parallel-banded variety of chalcedony, a silicate mineral. Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands; agate has curved bands while onyx has parallel bands. The colors of its bands range from black to almost every color. Specimens of onyx commonly contain bands of black and/or white. Onyx, as a descriptive term, has also been applied to parallel-banded varieties of alabaster, marble, calcite, obsidian, and opal, and misleadingly to materials with contorted banding, such as "cave onyx" and "Mexican onyx".

<span class="mw-page-title-main">Volcanic rock</span> Rock formed from lava erupted from a volcano

Volcanic rocks are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "pyroclastics," and these are also technically sedimentary rocks.

<span class="mw-page-title-main">Geode</span> Hollow formation inside a rock

A geode is a geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter are secluded. The crystals are formed by the filling of vesicles in volcanic and subvolcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same or other minerals precipitated from water, groundwater, or hydrothermal fluids.

<span class="mw-page-title-main">Chrysoprase</span> Gemstone variety of chalcedony

Chrysoprase, chrysophrase or chrysoprasus is a gemstone variety of chalcedony that contains small quantities of nickel. Its color is normally apple-green, but varies from turquoise-like cyan to deep green. The darker varieties of chrysoprase are also referred to as prase.

<span class="mw-page-title-main">Thunderegg</span> Nodule-like rock, that is formed within rhyolitic volcanic ash layers

A thunderegg is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. Thundereggs are rough spheres, most about the size of a baseball—though they can range from a little more than a centimeter to over a meter across. They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal, either uniquely or in combination. Also frequently encountered are quartz and gypsum crystals, as well as various other mineral growths and inclusions. Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colours. A characteristic feature of thundereggs is that the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.

<span class="mw-page-title-main">Dzi bead</span> Type of stone bead

A dzi bead a type of agate bead of uncertain origin found in the Himalayan regions including Tibet, Bhutan, and Ladakh.

<span class="mw-page-title-main">Harmotome</span> Rare zeolite mineral

Harmotome is a mineral, one of the rarer zeolites; a hydrated barium silicate with formula: (Ba0.5,Ca0.5,Na,K)5Al5,Si11O32·12(H2O). It forms vitreous white well defined monoclinic crystals, often associated with calcite and other zeolites. It has a Mohs hardness of 4 to 5 and a specific gravity of 2.44 to 2.5.

<span class="mw-page-title-main">Lithophysa</span> Felsic volcanic rock

A lithophysa is a felsic volcanic rock with a spherulitic structure and interior cavity with concentric chambers. Its outer shape is spherical or lenticular. They vary in size from very small up to twelve feet in diameter depending on the age of the magma chamber. These rocks are usually found within obsidian or rhyolite lava flows. Lavas low in feldspar minerals may produce a version known as snowflake obsidian.

<span class="mw-page-title-main">Moss agate</span> Agate variety

Moss agate is a semi-precious gemstone formed from silicon dioxide. It is a form of chalcedony which includes minerals of a green color embedded in the stone, forming filaments and other patterns suggestive of moss. The field is a clear or milky-white quartz, and the included minerals are mainly oxides of manganese or iron. It is not a true form of agate, because it does not have concentric banding.

<span class="mw-page-title-main">Quartz-porphyry</span> Type of volcanic rock containing large porphyritic crystals of quartz

Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz. These rocks are classified as hemi-crystalline acid rocks.

A range of gemstones are mentioned in the Bible, particularly in the Old Testament and the Book of Revelation. Much has been written about the precise identification of these stones, although largely speculative.

<span class="mw-page-title-main">Enhydro agate</span>

Enhydro agates are nodules, agates, or geodes with water trapped inside its cavity. Enhydros are closely related to fluid inclusions, but are composed of chalcedony. The formation of enhydros is still an ongoing process, with specimens dated back to the Eocene Epoch. They are commonly found in areas with volcanic rock.

<span class="mw-page-title-main">Fairburn Agate</span> Gemstone with concentric layers of cryptocrystalline chalcedony

The Fairburn Agate is a type of gemstone found in the agate beds of Southwestern South Dakota and Northwestern Nebraska. It is also the state gemstone of South Dakota. Fairburns are characterized from other types of agate by their colors and the shape of the bands.