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

• Aggos - marketplace name given thin slices of agates from the Yellowstone River area of eastern Montana that have been  honed to give them satin -- i.e., low matte-like, rather than polished -- surfaces.  Lapidaries think such surfaces better exhibit the agates' patterns and colors than polished surfaces do (Johnson et al., 1999, p.210).
• Amberine - yellowish green agate from Death Valley, California.
• Blood agate - blood-red, pink or salmon colored agate from Utah.  (cf. pigeon blood agate.)
  
• Brecciated agate - breccia the larger fragments of which are agate:  The names mosaic agate and ruin agate have also been given to some fractured and rehealed agate.
• Cloud agate - light gray and off-white chalcedony with an overall pattern that roughly resembles clouds.  Examples I have seen do not appear to fit any widely accepted definition of agate;  externally, the masses I have seen  resemble thunder eggs.
• Conachatae - agate with "inclusions of cacholong opal arranged as conic patches" (Mitchell, 1985).
• Condor agate - trade name for a rather "colorful [agate], good reds and browns, etc. ... from Patagonia" (Pough, personal communication, 1998).
• Dentritic agate - agate with inclusions or coatings of dendrites.  See also moss agate on this list.
  
• Dot agate - overall term given chalcedony that contains spheroids that appear as spots on plane surfaces;  see also Polka-dot agate on this list and St. Stephen's Stone under the OTHER NAMES subheading in the CHALCEDONY entry.
• Eye agate - agate with alternating, concentric color bands. The terms orbicular agate and Aleppo stone have also been applied to these agates, and agates with two "eyes" have been referred to as oxeye and owl-eye agates. (cf. ring agate.)  Many agates marketed as eye agate exhibit their "eyes" -- i.e., the concentric banding -- only because they have roughly spheroidal tops the diameters of which are perpendicular to the banding of the normal thin-banded agate rough from which they were fashioned;  this fact is quite apparent when one examines agate marbles (see, for example, www.marblealan.com/nonglass.).
  
• Fancy agate - term sometimes applied to agates exhibiting unusually intricate patterns.
• Fire agate (sometimes referred to as iridescent agate or rainbow agate) - to some people, this is a non-agate variety of chalcedony;  although it might be argued that its agate designation may have been based on the layered arrangement of the inclusions responsible for its iridescence, it seems more likely it was so-named as a marketplace ploy.  In any case, as the name suggests, some people see its overall appearance as resembling burning embers.  It is multicolored -- e.g., fine quality stones exhibit diverse combinations of red, violet, orange, green, yellow and brown hues.  As examples of fire agate localities, Sweaney (1979) mentions occurrences within the Sonora Desert region of northern Mexico and southern Arizona (e.g., on Saddle Mountain, near Tonopah, Maricopa County and near Safford, Graham County) and also from the Central Basin of Mexico. 
  
• Flame agate - Two quite different agates have been called flame agate in the literature.  Johnson and Koivula (1998) review and present fine illustrations of both. The following, with pertinent quotations from Johnson and Koivula (i.e., not directely from Sinkankas and Macpherson) briefly describe the appearances of these agates.

  1. John Sinkankas (1959) described flame agate as a "highly translucent, colorless agate with few typical agate bands, but rather containing long streaks or 'flames' of a bright red color" (see Johnson and Koivula, op cit., Figs. 4 and 5). The material he described came from Villa Ahumada, Chihuahua, Mexico.  

  2. H.G. Macpherson (1989) applied the term flame agate to an "agate of any color in which the pattern resembles a candle flame" (see Johnson and Koivula, op cit.,  Fig. 3).  Many agates from many worldwide localities, if correctly fashioned, could be designated as flame agate under this definition.

• Fortification agate - agate with bands that resemble bastions (etc.) of ancient forts -- cf. riband agate and ruin agate. Noteworthy fortification agate has been collected from Fourth of July Peak, near Hassayampa, Maricopa County, Arizona.
• Frost stone (or frost agate) - translucent gray chalcedony that contains sporadic snowflake-like masses.  A noteworthy occurrence is in the Mojave Desert of California.
• Hokkaido agate - overall "brown to gray nodular stone ... from the northernmost ... Japanese Islands.  The center of the stone has sharp fortification banding that blends into rounded bull's-eye banding around the outer protions... usually dyed to resemble [red to cinnamon-brown] carnelian (Messchært,1966-67).
• Iris agate - agate with extremely thin bands that apparently diffract light rays, like a diffraction grating does, thus exhibiting colors of the spectrum.  Iris agates have been recorded from several places in California; the vicinity of Challis, Custer County, Idaho; near Red Hill, Caron County, New Mexico; at Antelope, Waco County, Oregon; and along the banks of Yellowstone River, Montana.
• "Japanese jade" - name sometimes given "an opaque, nonvitreous, milky-white agate with spinach-green splotches (Messchært, 1966-67).
  
• Jasp agate (agate jasper, jasp fleuri, and jasponyx) - name sometimes given material that is considered either 1. intermediate between jasper and agate;   or  2.to consist of bands of transparent chalcedony and subtranslucent jasper.
• Lace agate - translucent blue and white banded chalcedony;  lace agate that resembles moonstone is referred to as blue lace agate in some marketplace outlets.  Three source areas are Imperial County, California, the Balmorhea area of Texas and Namibia (formerly South West Africa).  Also, a crazy lace agate has been recorded from several localities in the state of Chihuahua, Mexico.
• Laguna agate - trade name for "a very colorful Mexican agate" (Pough, personal communication, 1998) from, for example, Eijido Ojo Laguna, Chihuahua, Mexico.
• Lake Superior agate - name frequently given to agates found within upper Midwestern U.S. traprock masses of, for example,  Michigan's Keweenaw Peninsula  and in unconsolidated beach and glacial deposits derived from those rocks (Robinson, 2001). See also Shadow agate.
  
• Lantana - name broadly applied to banded agate, chalcedony and jasper found as beads produced in Ilorin, Nigeria during the 19th and 20th centuries (Lui, 1995).
• Mosaic agate - see Brecciated agate.  Unfortunately this term has also been applied to some brecciated "Mexican onyx" (see TRAVERTINE entry).
• Moss agate (Mocha stone or mocha stone) - subtransparent to subtranslucent, white, light gray, yellowish or greenish agate with green and/or brown to nearly red iron oxide and/or black manganese oxide arborescent inclusions that resemble moss, trees and/or landscapes. Historically, this designation moss agate has evolved as follows:  1.Appatently it was originally applied to chalcedony with green dendritic inclusions.  2.Subsequently, it was  extended to include chalcedony with reddish dendritic inclusions.  3.Later, it was extended further to include chalcedony with black dendrites, such as that previously called Montana agate.  [and]  4.Today, most, if of not all, chalcedony with any of these kinds of inclusions is widely referred to as moss agate.  Some agates of this genre, however, have been given additional adjectival modifiers that pertain to whatever their forms are thought to resemble -- e.g., bouquet, dendritic, flower, Indian, landscape, mosquito, painted, scenic, seaweed, tree, and even the less attractive adjectives gnat, midge and mosquito.  Diverse moss agates have long been associated with Hindustan;  Yellowstone National Park, Wyoming; and the bed of the Yellowstone River in Montana -- e.g., near Glendive, Dawson County, Montana.
• Occidental agate - term sometimes applied to low quality agate.
• Onyx (also onyx agate) - strictly speaking, coarsely banded agate -- see CHALCEDONY and ONYX entries.
• Pagoda stone - agate characterized by images that resemble pagodas.  Unfortunately, this designation has also been applied to other rocks and resin replicas of some of them that roughly resemble multi-story pagodas -- e.g., certain fossiliferous limestones (see MARBLE entry); differentially weathered thin-layered rocks, such as interlayered iron-rich and so-to-speak iron-free sandstones;  and so-called "stacked towers" of relatively flat stones.   In addition, a variety of jade (q.v.)  and the agalmatolite (see APPENDIX A), which is used in China for carving pagodas etc., are frequently called pagodaite. 
  
• Palm root agate - see FOSSILIFEROUS ROCKS entry.
• Pigeon blood agate - deep red and white agate from Cisco, Utah.
• Plume agate - a special kind of moss agate most of which consists largely of virtually colorless or light-colored, commonly bluish, agate with included  sporadic red, yellow, yellowish brown or violet fluffy-appearing masses, some of which resemble egret plumes. The "plumes" of these agates from a number of different localities have been variously said to consist of or pigmented by goethite, diverse manganese oxides or combinations thereof, realgar and/or orpiment. Some particularly well-studied plume agate occurs within  thunder eggs of Saguache County, Colorado (Kile, 2002, and see figure A in THUNDER EGG entry). Other fine examples have been collected near Prineville, Crook County, Oregon and south of Alpine, Brewster County, Texas.
• Polka dot agate - a translucent moss agate with small red, yellow or brown included masses that lend a polka dot appearance, especially on polished relatively flat surfaces.
• Pom pom agate - agate containing pufflike clusters.
• Riband agate (also called ribbon agate) - agate with relatively broad bands.
• Ring agate - agate exhibiting concentric color bands -- cf. eye agate.   In addition, however, the term "ring agate" is frequently used in the marketplace to refer to any agate that is set in any ring (finger, ear, etc.).
  
• Rogueite - milky white moss agate with green inclusions from the Rogue River valley of southwestern Oregon.
• Ruin agate - name applied variously to, for example, landscape agate that resembles ancient ruins and to brecciated agate.
• Saginitic agate - name applied to agates characterized as including "acicular, or needle-like, mineral growths" (see illustrations in McMahon, 1999).
• Shadow agate - agates of the Lake Superior region whose flat, polished surfaces appear wavy or to have shadowed areas, some of which have been termed "black holes."  Sukow (1999) offers an hypothesis to account for this feature, and indicates how one might spot this effect in raw material.
• Shell agate - misnomer applied to silicified mollusc shells -- see Turritella agate in FOSSILIFEROUS ROCKS entry.
• Snake skin agate - term applied to both silicified corals and chalcedony nodules whose surfaces roughly resemble the scaly pattern of some snakes' skins -- see FOSSILIFEROUS ROCKS entry.
• Star agate - agate that exhibits roughly star-shaped forms.
• Swiss lapis - marketplace name given to some agate that has been dyed blue to resemble lapis lazuli.
• Topographic agate - agate exhibiting lines that resemble contour lines on topographic maps.
• Vistaite - name given to moss agate and also to some of the jasper from the vicinity of Prineville, Crook County, Oregon.
• Wood agate - silicified wood that exhibits agatelike banding. Unfortunately, this term has also been applied, albeit incorrectly, to some silicified wood that is chalcedony and/or jasper with no banding similar to that which is characteristic of agate.
• Zigzag agate - agate, the polished surfaces of which exhibit a pattern of zigzag lines.

USES: Jewelry, 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.

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 (gemstone inlays)  since at least the late 17th century.

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.

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." 

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 and Prairie agate are, respectively, Nebraska's official state gem and state rock.  Agate, otherwise not specified, is one of two state gemstones of Montana.

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).

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R. V. Dietrich © 2008
Revised: November 1, 2008
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