agate

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agate

(ăg`ĭt), translucent, cryptocrystalline variety of quartzquartz,
one of the commonest of all rock-forming minerals and one of the most important constituents of the earth's crust. Chemically, it is silicon dioxide, SiO2.
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 and a subvariety of chalcedonychalcedony
[from Chalcedon], form of quartz the crystals of which are so minute that its crystalline structure cannot be seen except with the aid of a microscope. Chalcedony has a waxy luster and is translucent to transparent.
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. Agates are identical in chemical structure to jasper, flint, chert, petrified wood, and tiger's-eye, and are often found in association with opal. The colorful, banded rocks are used as a semiprecious gemstone and in the manufacture of grinding equipment. An agate's banding forms as silica from solution is slowly deposited into cavities and veins in older rock. The stones can be artificially stained to produce combinations of color more vivid than those found in the natural state. The cutting and staining of agates has long been centered at Idar-Oberstein, Germany. Important sources of agate are Brazil, Uruguay, and the United States (Oregon, Washington, and around Lake Superior). The moss agate or mocha stone contains visible impurities in the form of dendritic shapes that resemble moss. See onyxonyx
, variety of cryptocrystalline quartz, differing from agate only in that the bands of which it is composed are parallel and regular. Its appearance is most striking when the bands are of sharply contrasting colors; black and white specimens are often used for cameos.
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The Columbia Electronic Encyclopedia™ Copyright © 2013, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Agate

 

a mineral; a variety of chalcedony in the form of a dense, cryptocrystalline aggregate of fibrous or radial quartz formations (Si02)—so-called quartzite or α-tridymite. The mineral is characterized by multiple interstratification of thin (up to 10 microns), variously colored layers. The hardness of agate on the mineralogical scale is 6.0–6.5. The following varieties are recognized depending on the combination of colors in the layers—onyx (white and black layers), carnelian (red and white), sardonyx (reddish brown and white), and agates (bluish gray and white). Agate occurs in veins, in geodes, in amygdules among igneous rocks, and in the tuffs of the last. It is used as an industrial or semiprecious stone and in the manufacture of technical articles (bearing stones, prisms for scales, and so on). Agate is colored artificially to obtain bright, decorative forms.

G. P. BARSANOV

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

agate

[′ag·ət]
(graphic arts)
A type size in printing of about 5½ points, where 72 points equals 1 inch.
(mineralogy)
SiO2 A fine-grained, fibrous variety of chalcedony with color banding or irregular clouding.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

agate

symbolizes health; supposed to relieve snake and scorpion bites. [Class. and Medieval Legend: Leach, 27]
See: Health

agate

confers this power. [Rom. Folklore: Brewer Dictionary,15]
Allusions—Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. Copyright 2008 The Gale Group, Inc. All rights reserved.

agate

1. an impure microcrystalline form of quartz consisting of a variegated, usually banded chalcedony, used as a gemstone and in making pestles and mortars, burnishers, and polishers. Formula: SiO2
2. a playing marble of this quartz or resembling it
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
The Gitchee Gumee Agate and History Museum in Grand Marais owns a collection of marbles made from Lake Superior agates.
The museum also offers rock-hounding classes, where participants begin at the museum and learn about identifying agates, then head to the shores of Lake Superior to search for the stones.
A 1902 article in the Detroit Free Press stated that many imitation agate marbles "that [wore] holes in the pockets of schoolboys" were made in Germany.
Examples of Brazilian agate and onyx cut into bookends can be found at TheRockShed.com.
Swirls of natural graphic design make a group of agate ornaments intriguing for the holiday tree, or just to hang on cupboards or window latches.
Target's fall collection includes the Threshold agate bookend, sleekly honed on one end to show the swirling layers, and left in its natural state on the other.
Agates have been prized since ancient times and were actually named by the Ancient Greeks after the river Achates where they were first found.
At first, from the 1400s onwards, the cutters in Idar-Oberstein used locally-sourced material found along the Nahe river but when supplies ran out in the 19th century they started importing agates from Brazil and elsewhere.
There are currently over 100 different varieties of agate from Mexico; a number of observable features which are characteristic of the various types of banded agates are key in distinguishing one variety from another.
The agates from this region all originate in what is locally known as the Rancho El Agate andesite, a 38-million-year-old porphyritic andesite.
Agate, formed when mineral-rich water flows through volcanic rock, consists of millions of micrometer-sized crystals.
Observing agate slices with transmission electron microscopy and ion mass spectroscopy, the two scientists found that the size of the tiny crystals and the degree of impurities change cyclically, forming the iris band's crystal pattern.