AGATE

(Fr- agate; Ger- Achat; Nor-agat; Rus-)

AGATE, SiO2. (See also CHALCEDONY, ONYX and THUNDER EGG entries.)


A. Agate, polished surface of a Lake Superior agate (width  ~ 8 cm) from Keweenaw Point, Michigan.  Specimen collected, cut and polished by Robert J. Barron.  Seaman Museum, Michigan Technological University.  (© photo by John Jaszczak) 

B. Fire Agate (greater dimension - 2.1 cm) from Deer Creek, Arizona.  Rincon Mineral Co. (© photo by Jeffrey A. Scovil)

C. Iris agate (width - 9 cm) from Patagonia, Argentina.  St. Paul Gems & Minerals. (© photo by Jeffrey A. Scovil)

D. Agate mouse (height - ca. 6 cm) carved by Gerd Dreher.  Silverhorn. (© photo by Jeffrey A. Scovil)

E. "Joshua tree agate" (width - 6.4 cm).  Jimmy Vacek collection. (© photo by Jeffrey A. Scovil)

F. Moss agate cabochon (greater dimension - 4.8 cm) from India.  Zeolites India. (© photo by Jeffrey A. Scovil)

G. Agate (width - 28.4 cm) from Rio Grande do Sul, Brazil, that has been dyed with diverse hues.  Frederick H. Pough collection. (© photo by Jeffrey A. Scovil)

H. Agate from Idar-Oberstein, Germany -- see photograph, full caption and related text  under REMARKS subheading.

[See also Figures 51-59 and AppB1-AppB10 in the MIMETOLITH file on this web site.]

DESCRIPTION: Primarily, agate is conspicuously banded chalcedony;  the bands range from parallel to highly uneven, even within some  individual specimens.  Both color and light transmission commonly differ from band to band.  In addition, chalcedony with variously shaped splotchy areas, commonly having arborescent shapes are called agate, usually moss agate.  
    Colors - reddish brown, yellowish brown, green (rare), blue (rare), purplish, white (including milky and bluish white), gray, greenish gray, black, and brown
    H. 6½-7
    S.G. 2.59-2.67
    Light transmission - transparent to subtranslucent (cf. jasper)

    Luster
- waxy

    Breakage - subconchoidal fracture yielding fine granular appearing surfaces
    Miscellaneous - commonly triboluminescent.

OTHER NAMES: Several names have been introduced.  Most consist of adjectives based on characteristic features, localities or fancies plus agate -- e.g., fire agate.  In addition, agate and agatized have been applied, albeit inappropriately, to largely chalcedony materials that are neither banded nor moss agate. Pabian (2002) lists what he calls "2,943 [updated as of 3/12/'01 to 3,084] named varieties," which, as noted in the lexicon, actually includes names given "Agate, Jasper, Opal and other Amorphous, Cryptocrystalline, or Spherulitic Forms of Gem Silicon Dioxide."  In any case, many terms have been given diverse agates, especially in  rock shops and advertisements.  And, in my opinion, most of these terms seem ill-conceived.  Examples of names, most of which have been circulated relatively widely,  follow:

USES: Jewelry, buttons, letter openers, handles for cutlery, paper weights, bookends, picture frames, giutar picks, bowls, coasters,  etc.  High quality cameos and intaglios and other engravings fashioned from agate (see also ONYX entry) have been used in such diverse pieces as earrings, brooches, pendants, plaques and tietacks -- see the fine example pictured on the cover of the spring issue of Gems & Gemology, volume 19.  One of the most interesting pieces fashioned from agate that I know about is the head of a Scepter from Cyprus, now in the Metropolitan Museum of Art at New York City. 

An agate tool found beneath volcan ash erupted from Mount St. Helens ~15,800 ago appears to support the existence of humans more ancient than the Clovis culture, which is frequently been cited as the oldest human group of the western United states.  This tool, which has been found to have blood residue of an extinct Bison on it, is said to be a product of the University of Oregon's archaeolotical field studies. (Bodman, 2015)

Eye agates, especially those with dark centers, were used for the eyes in carvings of idols, particularly in ancient Egypt.

Spendlove (1979) illustrates agates – e.g., ruin agates – upon which art work has been painted.  Painters have also used other agates as backgrounds for their paintings.  

Chalcedony, especially agate, has been used along with other gemrocks, such as lapis lazuli, malachite and rhodochrosite, in the production of intarsias (i.e., gemstone inlays) since at least the late17th century.   In addition, dendritic agates have been incorporated in doublets in jewelry -- e.g., "Coral sea" agate (q.v.) -- and also as "discs framed inside of other gem materials such as rock crystal, jasper, chrysoprase, obsidian, tiger's-eye, and banded agate" (Overlin, 2011) and elsewise (ibid)

Two other uses of agates, not as a gemrock but of possible practicable interest, are for the production of mortars and pestles and the fulcrums for balances.

Perhaps one of the best known uses of agate, especially in the past but persisting today, is as the small spheres used for "shooting marbles." Indeed, some people call marbles (mibs) -- including those not made of agate -- "agates."

OCCURRENCES: Agates most commonly occur as cavity fillings (nodules) in consolidated basaltic magmas and in detrital material (e.g., alluvium, beach gravels and glacial deposits) derived from those rocks. Noteworthy quantities also occur as so-called replacement fossils -- e.g., agatized wood.

NOTEWORTHY LOCALITIES: Much of the color-banded agate of the marketplace has come from a roughly belt-shaped region that extends from near Salto, in eastern Argentina, to near Porto Alegre, Rio Grande do Sun, southern Brazil.  Noteworthy quantities have also come from the Deccan "trap rocks" (i.e., basaltic rocks) of India;  from a number of regions in China;  and from the Agate Creek area of North Queensland, Australia.  Several diverse kinds of agates have been found at literally scores of localities in North America -- see, for example, those noted in the articles in the book compiled and edited by Leiper (1966). See also those noted after some of the agates listed under the OTHER NAMES subheading.  Some of the agates that occur as amygdules within or that have been weathered out of  "Lake Superior meta-basalts" in northwestern Michigan are of particular interest because of their inclusions of diverse minerals.  These minerals, which occur either individually, paired, etc., are well described and illustrated by Rosemeyer (2012).  The minerals that occur are native copper (apparently most common), chamosite, epidote,  hematite, cuprite, and prehnite and calcite (ibid.).
            Agates with diverse patterns, which are recorded as consisting of "three silica phases: low-a quartz, moganite [non-approved term] and opal-CT,"  and occur within Triassic "basaltoids" in Sidi Rahal of the Morocco Atlas mountains are described and illustrated by Dumanska-Slowik et al., 2014.

Another possible locality(?):  David Livingstone, the famous Scottish missionary and African explorer who discovered Victoria Falls in 1855, wrote (1865), "The ground is strewn with agates for a number of miles above the falls."  According to information Craig Gibson (personal communications and enclosures sent 2005) agate amygdules do occur in basaltic lavas that crop out at Victoria Falls;  however, he also notes, "I have never heard of the agates from the Falls area being used in any commercial manner and I am sure that if they had been that would have been mentioned in the literature." 
REMARKS:Theophrastus in his well-known On Stones  (ca. 315 B.C.) indicates that the designation agate was based on the source of such stones from the Achates (now Drillo) River in southwestern Sicily, which, by the way, is still a source of agate and chalcedony.  Be that as it may, between 3000 and 2300 B.C., Sumerains (and apparently even earlier, Babylonians) fashioned agate into axe heads, some of only ceremonial use.

Many color-banded agates of the marketplace have been dyed.  Indeed, agate can be dyed almost any color or black by soaking it in various chemical solutions (see Fig. G). (It is surprising that someone has not differentially dyed, for example, fortification or ruin agates, to distinquish what might be interpreted as ground versus sky, etc. -- Perhaps someone has and I have just not seem the results.)  In addition, heat treatments have been used -- both alone and along with, for example, immersion in certain chemicals -- to change agates' colors.

Agate per se has long been reputed to prevent or help cure all sorts of disorders -- e.g., to prevent insomnia and even as a follow-up to promote pleasant dreams.  In addition, agate has been said to make people more persuasive and less disagreeable.  Also, eye agate amulets have been, and in some places continue to be, attributed the power of curing skin diseases;  but, elsewhere their "fixed stare" is said to have raised all kinds of reprehensible reactions, including those associated with the proverbial "evil eye."  In addition, moss agate also has been given several health-related attributes, especially those related to people's eyes and mouths.  --  A list of legends associated with agate could go on and on ... and on.  The following lines of poetry about moss agate, said to have been written in the 4th century A.D. (King, 1865), serve, I think, as a fitting closing for this paragraph:

"Carrying the Tree-Stone with thee to the shrine,
  Thou shalt propitiate each power divine.
  The gem the semblance of a garden shows,
  Where growing trees entwine their leafy boughs;
  Hence a fit title bears it with the wise
  Who the Tree-Agate is a treasure prize:
  One part displays the perfect Agate-stone,
  In one a shaggy grove is plainly shown."




H. Agate slice (greater dimension - 13 cm) from Baumholder, near Idar-Oberstein, Rhineland-Palatinate, Germany -- see the following paragraph for additional information.  Donald Gabriel Collection (DCG 355) in Seaman Museum, Michigan Technological University.  (© photo by George Robinson)  

Agates, in lava flows in the vicinity of Idar-Oberstein, southwestern Germany, served to initiate the stone cutting industry that led to these towns' becoming one of the world's largest centers for cutting and polishing colored stones.  Foshag (1953) notes that "The Idar-Oberstein agate is a distinctive type .... It is, generally, more colorful than agates from other areas; many show a rich, red color delicately mottled or spotted. Other types show a sharp narrow zoning of dense layers. A common characteristic is an outer zone of dense, finely banded agate with a central filling of crystalline quartz or amethyst. Although many small amygdules of agate can be seen in the basalt cliffs [of the area], pieces of usable size were and are now extremely rare."  George Robinson (personal communication, September, 2004) notes of the specimen shown as "H":   "Its colors are incredible (and natural!) and it is perhaps the finest agate in our [theSeaman Museum] collection." 

Agate was apparently the first stone in Aaron's breastplate (see GLOSSARY) and is thought by some scholars to have occupied position number nine in High Priests' breastplates.  Agate is considered by some people to be the birthstone for June, and by others to relate to the zodiac symbol Gemini (May 21 - June 21).

A rather interesting aspect of recovery of dendritic agates in central India is reported by Weldon (2008):  "Miners work the riverbed twice a year during the region's dry seasons.  One unusual approach taken by local miners is to plant cucumbers and other deep-rooting vines in the riverbed, where the fast-growing roots reportedly loosen the soft alluvium, causing many nodules to rise to the surface.  These are then easily collected for cutting."

The Fairburn agate, also called fairburnite is the official state gemstone of South Dakota;  individual specimens of these agates, which are fine examples of fortification and riband agates, occur sporadically in gravels in the Black Hills of South Dakota (Clark, R.W., 1998).  Agate referred to as Lake Superior or Duluth agate in Minnesota, and sometimes called Keweenaw or Isle Royale agate in Michigan, is the official state gemstone of Minnesota.  Blue agate (I have been told, but have not been able to confirm this, that chalcedony is what actually has this distinction.) and Prairie agate are, respectively, Nebraska's official state gem and state rock.  Agate found in gravel is the state gemstone of Louisiana.  Agate, otherwise not specified, is one of two state gemstones of Montana.  Agatized coral is the state stone of Florida.

SIMULANTS:

***Agate ware - Wedgwood china made to roughly resemble agate. - [Appearance is sufficient to distinguish this material from agate.].

Brecciated travertine - this gemrock is sometimes marketed as mosaic agate. - [inferior hardness].

***Ceramic ware with enclosed horsehair --e.g., some off-white ceramic bowls (etc.), whose clay precursor had horsehair added to it prior to its being fired, that roughly resemble moss agate.  I have seen only photographs of these advertized as having been made by "Acoma Pueblo artists." - [Appearance suffices to distinguish this material from moss agate.].

***Glass - Marilyn Jobe of Ellenton, Florida has produced glass, fashioned as beads and cabochons, that closely resembles agate - [inferior hardness (H. ~ 5)].

Imposed (etc.) dendrites  -  1.a somewhat complicated procedure has been used to produce native copper dendrites on chalcedony (Koivula, 1986, p.246);    2.the dendritic inclusions of some plume agates have been produced by electrically stimulated chemical precipitation and thence incorporated as parts of assembled stones;  [and]  3.a chalcedony cabochon with a dendritic-shaped pattern painted on it to give it a moss agate appearance has been recorded (Kane, 1988).

***Plastic - a photograph of a "plastic imitation simulating agate" is shown by Nassau (1974). - [inferior hardness (H. < 5)].

Royal agate - term sometimes given to mottled obsidian. - [inferior hardness (H. 5-5½)].

***Stratified Pyrex - pyrex glass with layers of more than one color. - [inferior hardness (H. ~ 5)].

Whale molar (dyed) - some beads of this material closely resemble agate; Liu (1994, p.74) illustrates a couple of these beads that have been termed "crane's crest" - [texture and inferior hardness (H. ~ 3)].

REFERENCES: Cross, 1998;  Dake, Fleecer and Wilson, 1938;  Frondel, 1962;  Leiper, 1966;  Macpherson, 1989.  (For information re dendrites, see Potter and Rossman, 1979A).  Rocks & Minerals -- Vol. 89, No.4 includes a number of well illustrated articles about agates -- e.g., those from southeastern Utah and Keweenaw County, Michigan -- as well as an article about some thinsections of agates from localities in, for example, Brazil and France, AND an short feature about an artist's depiction of agates.

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Last update: 10 March 2015
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