agate; Ger- Achat;
A. Agate, polished surface of a Lake
Superior agate (width ~ 8 cm) from Keweenaw Point,
Michigan. Specimen collected, cut and polished by Robert J.
Seaman Museum, Michigan Technological University. (© photo
by John Jaszczak)
B. Fire Agate (greater
dimension - 2.1 cm) from Deer Creek,
Rincon Mineral Co. (© photo by
Jeffrey A. Scovil)
C. Iris agate (width - 9 cm) from
Patagonia, Argentina. St. Paul Gems & Minerals. (©
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. (©
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
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
OTHER NAMES: Several
names have been
introduced. Most consist of adjectives
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:
Colors - reddish brown, yellowish brown,
green (rare), blue (rare), purplish, white
(including milky and bluish white), gray, greenish gray, black,
Light transmission - transparent to
subtranslucent (cf. jasper)
Luster - waxy
Breakage - subconchoidal
fracture yielding fine granular appearing surfaces
- 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
- 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
any widely accepted definition of agate; externally, the masses I
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).
sea" agate - trade name for a doublet of lace agate atop reconsituted
turquoise (Overton, 2012).
- 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;
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
"eyes" have been referred to as oxeye
and owl-eye agates. (cf. ring
agate.) Many agates marketed as eye agate exhibit their "eyes" --
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,
- 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
marketplace ploy. In any case, as the name suggests, some people
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
- Flame agate - Two quite different agates have
been called flame agate in the literature.
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
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
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
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.
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
- Mosaic agate - see Brecciated agate.
Unfortunately this term has also been applied to some brecciated
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
2.Subsequently, it was extended to include chalcedony with
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.
- Mushroom agate - cabochons,
beads and pendants have been marketed widely as as mushroom agate,
mushroom jasper and mushroom rhyolite, which is said to have been
metamorphosed. All, if indeed three differently
constituted ones exist, are said to come from Arizona. I have
seen only photographs of any of these, and all appear to be
the same material. In any case, the photographs do exhibit
that resemble mushrooms on polished surfaces.
- Occidental agate
- term sometimes applied to low quality agate.
- Onyx (also onyx agate) - strictly speaking,
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
- Palm root agate - see FOSSILIFEROUS ROCKS
- 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
- 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
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
- 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,
- 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
- Zigzag agate - agate, the polished surfaces of
which exhibit a pattern of zigzag lines.
USES: Jewelry, buttons,
letter openers, handles for
cutlery, paper weights, bookends, picture frames, giutar
picks, bowls, coasters, etc.
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
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,
Eye agates, especially those with dark centers,
were used for the eyes in carvings of idols, particularly in ancient
Spendlove (1979) illustrates agates – e.g., ruin
– 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)
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
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,
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
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
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,
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.
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
The gem the semblance of a garden
Where growing trees entwine their
Hence a fit title bears it with
Who the Tree-Agate is a treasure
One part displays the perfect Agate-stone,
In one a shaggy grove is plainly
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
central filling of crystalline quartz or amethyst. Although many small
amygdules of agate can be seen in the basalt cliffs [of the area],
of usable size were and are now extremely rare." George Robinson
communication, September, 2004) notes of the specimen shown as "H":
"Its colors are incredible (and natural!) and it is perhaps the
agate in our [theSeaman Museum] collection."
Agate was apparently the first stone in Aaron's
(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
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
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
Montana. Agatized coral is
the state stone of Florida.
***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
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
- [inferior hardness (H. ~ 5)].
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
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.
Royal agate - term sometimes given to mottled
obsidian. - [inferior hardness (H.
***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
Dake, Fleecer and Wilson, 1938; Frondel, 1962; Leiper,
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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