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Encyclopedia > Agate
Agate

Moss agate pebble, 2.5 cm (1 inch) long
General
Category Mineral
Chemical formula Silica, SiO2
Identification
Color White to grey, light blue, orange to red, black.
Crystal habit Cryptocrystalline silica
Crystal system Rhombohedral Microcrystalline
Cleavage None
Fracture Conchoidal with very sharp edges.
Mohs Scale hardness 7
Luster Waxy
Refractive index 1.530-1.540
Birefringence up to +0.004 (B-G)
Pleochroism Absent
Streak White
Specific gravity 2.58-2.64

Agate is a type of quartz (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 but can be common in certain metamorphic rocks.[1] Image File history File links This is a lossless scalable vector image. ... Look up agate in Wiktionary, the free dictionary. ... Download high resolution version (750x750, 99 KB)Moss agate pebble, one inch across (2. ... For other uses, see Mineral (disambiguation). ... A chemical formula is a concise way of expressing information about the atoms that constitute a particular chemical compound. ... In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. ... A cryptocrystal is a rock whose texture is so finely crystalline—that is, made up of such minute crystals—that its crystalline nature is only vaguely revealed even in a thin section by transmitted polarized light. ... A crystal system is a category of space groups, which characterize symmetry of structures in three dimensions with translational symmetry in three directions, having a discrete class of point groups. ... In crystallography, the rhombohedral (or trigonal) crystal system is one of the 7 lattice point groups. ... Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite planes, creating smooth surfaces, of which there are several named types: Basal cleavage: cleavage parallel to the base of a crystal, or to the plane of the lateral axes. ... For fractures in geologic formations, see Rock fracture. ... A conchoidal fracture is produced when some types of fine-grained mineral, such as obsidian and flint, are broken. ... Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer. ... Lustre (American English: luster) is a description of the way light interacts with the surface of a crystal, rock or mineral. ... The refractive index (or index of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. ... A calcite crystal laid upon a paper with some letters showing the double refraction Birefringence, or double refraction, is the decomposition of a ray of light into two rays (the ordinary ray and the extraordinary ray) when it passes through certain types of material, such as calcite crystals, depending on... Pleochroism is an optical phenomenon in which grains of a rock appear to be different colors when observed at different angles,under a petrographic microscope. ... The streak (also called powder color) of a mineral is the color of the powder produced when it is dragged across a unweathered surface. ... Relative density (also known as specific gravity) is a measure of the density of a material. ... R-phrases R42 R43 R49 S-phrases S22 S36 S37 S45 S53 Flash point non-flammable Supplementary data page Structure and properties n, εr, etc. ... Chalcedony knife, AD 1000-1200 Bloodstone redirects here. ... Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means change in form, derived from the Greek words meta, change, and morphe, form. The protolith is subjected to extreme heat (>150 degrees Celsius) and pressure causing profound...


The colorful agate and other chalcedonies were obtained over 3,000 years ago from the Achates River, now the Drillo, in Sicily.[2] Chalcedony knife, AD 1000-1200 Bloodstone redirects here. ... Sicily ( in Italian and Sicilian) is an autonomous region of Italy and the largest island in the Mediterranean Sea, with an area of 25,708 km² (9,926 sq. ...

Contents

Formation and characteristics

Most agates occur as nodules in volcanic rocks or ancient lavas where they represent cavities originally produced by the disengagement of volatiles in the molten mass which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. 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. A nodule in petrology or mineralogy is an irregular rounded to spherical concretion. ... This article is about volcanoes in geology. ... Look up lava, Aa, pahoehoe in Wiktionary, the free dictionary. ... Volatiles are that group of compounds with low boiling points (see volatile) that are associated with a planets or moons crust and/or atmosphere. ...


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 vapour-vesicles. Variations in the character of the solution or in the conditions of deposit may cause 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 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 67 tons was exhibited at the Dusseldorf Exhibition of 1902. Perhaps the most comprehensive review of agate chemistry is a recent text by Moxon cited below. The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2. ... For other uses, see Quartz (disambiguation). ... For other uses, see Viscosity (disambiguation). ... Geode, halved and polished Geode, halved and polished Geodes (Greek geoides, earthlike) are geological rock formations which occur in sedimentary and certain volcanic rocks. ... For other uses, see Amethyst (disambiguation). ...


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. Celadonite is a mica group mineral, a phyllosilicate of potassium, iron in both oxidation states, aluminium and hydroxide with formula: K(Mg,Fe2+)(Fe3+,Al)[Si4O10](OH)2. ... Delessite is a mineral, a magnesium rich form of Chamosite which is, itself, a member of the Chlorite group Category: ... For other uses, see Iron (disambiguation). ... Augite is a single chain inosilicate mineral described chemically as (Ca,Mg,Fe)SiO3 or calcium magnesium iron silicate. ... Iron oxide pigment There are a number of iron oxides: Iron oxides Iron(II) oxide or ferrous oxide (FeO) The black-coloured powder in particular can cause explosions as it readily ignites. ... Limonite Limonite Limonite is a ferric hydrate of varying composition, the generic formula is frequently written as FeO(OH)·nH2O, although this is not entirely accurate as Limonite often contains a varying amount of oxide compared to hydroxide. ...

Banded agate (agate-like onyx). The specimen is 2.5 cm (1 inch) wide.

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 quartz, often amethyst, having the apices of the crystals directed towards the free space so as to form a crystal-lined cavity, or geode. Image File history File links Download high resolution version (750x626, 88 KB) Banded agate. ... Image File history File links Download high resolution version (750x626, 88 KB) Banded agate. ... Onyx is a cryptocrystalline form of quartz. ... For other uses, see Crystal (disambiguation). ...


On the disintegration of the matrix in which the agates are embedded, 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 shorelines. Loess field in Germany Surface-water-gley developed in glacial till, Northern Ireland Technically, soil forms the pedosphere: the interface between the lithosphere (rocky part of the planet) and the biosphere, atmosphere, and hydrosphere. ... Gravel (largest fragment in this photo is about 4 cm) Gravel is rock that is of a certain particle size range. ...


Types of agate

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 (named varieties include Maury Mountain, Richardson Ranch, Sheep Creek and others). Dendritic agates have beautiful fern like patterns on 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. A Mexican agate, showing only a single eye, has received the name of cyclops. ... Moss agate pebble, 1 inch (25 mm) long. ...


Turritella agate is formed from fossil Turritella shells silicified in a chalcedony base. Turritella are spiral marine 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 agate or stone.m For other uses, see Fossil (disambiguation). ... Subclass Subclass Eogastropoda     Patellogastropoda Subclass Orthogastropoda   Superorder Cocculiniformia   Superorder Hot Vent Taxa     Neomphaolida   Superorder Vetigastropoda   Superorder Neritaemorphi     Neritopsina   Superorder Caenogastropoda     Architaenioglossa     Sorbeoconcha   Superorder Heterobranchia     Heterostropha     Opisthobranchia     Pulmonata The gastropods, or univalves, are the largest and most successful class of mollusks, with 60,000-75,000 species, and second largest class... Extant Subclasses and Orders Alcyonaria    Alcyonacea    Helioporacea Zoantharia    Antipatharia    Corallimorpharia    Scleractinia    Zoanthidea [1][2]  See Anthozoa for details For other uses, see Coral (disambiguation). ... Petrified log at the Petrified Forest National Park A petrified tree from California Petrified wood is a type of fossil: it consists of fossil wood where all the organic materials have been replaced with minerals (most often a silicate, such as quartz), while retaining the original structure of the wood. ... A Petoskey stone A Petoskey stone is a rock, often pebble-shaped, that is composed of a fossilized coral, Hexagonaria percarinata. ...

Montana Moss Agate

Greek agate is a name given to pale white to tan colored agate found in Sicily back to 400 B.C. The Greeks used it fo making jewelry and beads. Today any agate of this colour from Greek/Sicily area of the Mediterranean is called Greek agate. Image File history File links Download high-resolution version (2304x1728, 567 KB) Brooks Britt Specimen and Photo I, the creator of this work, hereby release it into the public domain. ... Image File history File links Download high-resolution version (2304x1728, 567 KB) Brooks Britt Specimen and Photo I, the creator of this work, hereby release it into the public domain. ...


Another type of agate is Brazilian agate, which is found as sizeable 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 carnelian agate (usually exhibiting reddish hues), Botswana agate, Ellensburg blue agate, blue lace agate, plume agate (such as Carey, Graveyard Point, Sage, St. Johns, Teeter Ranch and others), tube agate (with visible flow channels), fortification agate (which exhibit little or no layered structure), fire agate (which seems to glow internally like an opal) and Mexican crazy-lace agate (which exhibits an often brightly colored, complex banded pattern).shtc


Agate beliefs

Faceted Botswana agate

In many traditions agate is believed to cure the stings of scorpions and the bites of snakes, soothe the mind, prevent contagion, still thunder and lightning, promote eloquence, secure the favour of the powerful, and bring victory over enemies.[citation needed] Persian magi are also known to have prized agate rings in their work and beliefs. Image File history File links Agatebots. ... Image File history File links Agatebots. ... Superfamilies Pseudochactoidea Buthoidea Chaeriloidea Chactoidea Iuroidea Scorpionoidea See classification for families. ... For other uses, see Snake (disambiguation). ... This article is about the medical term. ... Thunder is the sound made by lightning. ... For information on lightning precautions, see Lightning safety. ... Eloquence (from Latin eloquentia) is fluent, forcible, elegant or persuasive speaking in public. ... For other uses, see Magi (disambiguation). ...


Some followers of Pagan religions also believe agate is a crystal whose powers can be used for love, mental clarity, and good luck in card games.


Shia Muslims often wear an agate ring on the right hand, the stone engraved with the name of Allah, Ali, or the names of the other eleven Imams. It is known as "aqaq" or "agag" in Persian. Shiʻa Islam (Arabic شيعى follower; English has traditionally used Shiite) makes up the second largest sect of believers in Islam, constituting about 30%–35% of all Muslim. ... Allah is the Arabic language word for God. ... For other uses, see Ali (disambiguation). ... This article does not cite any references or sources. ...


Uses in industry

Industry uses agates chiefly to make ornaments such as pins, brooches, paper knives, inkstands, and seals. 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.


See also

Gem animals. ... Wikipedia does not have an article with this exact name. ... Fire Agate is a layered stone. ...

Notes

  1. ^ Donald W. Hyndman, David D. Alt (2002). Roadside Geology of Oregon, 18th, Missoula, Montanat: Mountain Press Publishing Company, p. 286. ISBN 0-87842-063-0. 
  2. ^ Agate Creek Agate. Retrieved on 2207-07-01.

The 23rd century (Gregorian Calendar) comprises the years 2201-2300. ... is the 182nd day of the year (183rd in leap years) in the Gregorian calendar. ...

References

  • The Nomenclature of Silica by Gilbert Hart, American Mineralogist, Volume 12, pages 383-395, 1927
  • International Colored Gemstone Association
  • Mindat data
  • Schumann, Walter. Gemstones of the World. 3rd edition. New York: Sterling, 2006.
  • http://www.gemstone.org/gem-by-gem/english/agate.html
  • 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.
  • Cross, Brad L. and Zeitner, June Culp. "Geodes. Nature's Treasures". Bardwin Park, California, Gem Guides Book Co. 2005.

  Results from FactBites:
 
Agate (1967 words)
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 vapour-vesicles.
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 quartz, often amethystine, having the apices of the crystals directed towards the free space, so as to form a crystal-lined cavity or geode.
On the disintegration of the matrix in which the agates are embedded, they are set free, and, being by their siliceous nature extremely resistant to the action of air and water, remain as nodules in the soil and gravel, or become rolled as pebbles in the streams.
Agate - Wikipedia, the free encyclopedia (844 words)
Most agates occur as nodules in volcanic rocks or ancient lavas where they represent cavities originally produced by the disengagement of volatiles in the molten mass which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls.
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 quartz, often amethyst, having the apices of the crystals directed towards the free space so as to form a crystal-lined cavity, or geode.
The agates are extremely resistant to weathering and remain as nodules in the soil or are deposited as gravel in streams and shorelines.
  More results at FactBites »

 
 

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