Agate
Agate // is a
microcrystalline variety of
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 and can be common in certain
metamorphic rocks.
[1]
Etymology and history
The stone was given its name by
Theophrastus, a Greek philosopher and naturalist, who discovered the stone along the shore line of the river
Achates (
Greek:
Ἀχάτης) sometime between the 4th and 3rd centuries BC.
[2] Colorful agates and other
chalcedonies were obtained over 3,000 years ago from the Achates River, now called
Dirillo, in
Sicily.
[3]
Ancient use
Agate is one of the most common materials used in the art of
hardstone carving,
and has been recovered at a number of ancient sites, indicating its
widespread use in the ancient world; for example, archaeological
recovery at the
Knossos site on
Crete illustrates its role in
Bronze Age Minoan culture.
[4]
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. Agate has
also been known to fill veins or cracks in volcanic or altered rock
underlain by granitic intrusive masses. 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.
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 deposition may
cause a 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 still
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 35 tons was exhibited at the
Düsseldorf Exhibition of 1902. Perhaps the most comprehensive review of agate chemistry is a recent text by Moxon cited below.
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.
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
drusy quartz, sometimes 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. The agates are extremely resistant to weathering and
remain as nodules in the
soil or are deposited as
gravel in streams and shorelines.
Types of agate
"Turritella agate" (
Elimia tenera) from Green River Formation, Wyoming
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.
Dendritic
agates have fern like patterns in 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.
Turritella agate is formed from silicified
fossil Elimia tenera (erroneously considered
Turritella) shells.
E. tenera are spiral freshwater
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 stone or 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 for making jewelry and
beads. Even though the stone had been around centuries and was known to
both the Sumerians and the Egyptians, both who used the gem for
decoration and for playing important parts in their religious
ceremonies, any agate of this color from Sicily, once an ancient Greek
colony, is called Greek agate.
Another type of agate is Brazilian agate, which is found as sizable
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
Lake Superior agate,
carnelian agate (exhibiting reddish hues), Botswana agate, blue lace
agate, plume agates, moss agate, tube agate (with visible flow channels
or pinhole-sized 'tubes'), fortification agate (which exhibit little or
no banding structure),
fire agate (which has internal flash or
'fire', the result of a layer of clear agate over a layer of
hydrothermally-deposited hematite), Mexican crazy-lace agate, which
often exhibits a brightly colored, complexly banded pattern (also called
Rodeo Agate and Rosetta Stone depending on who owned the mine at the
time).
The "Rubens Vase" (
Byzantine Empire).
Carved in high relief from a single piece of agate, this extraordinary
vase was most likely created in an imperial workshop for a Byzantine
emperor.
Uses in industry and art
Industry uses agates chiefly to make ornaments such as pins, brooches
or other types of jewelry, paper knives, inkstands, marbles and seals.
Agate is also still used today for decorative displays, cabochons,
beads, carvings and
Intarsia
art as well as face-polished and tumble-polished specimens of varying
size and origin. 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.
Idar-Oberstein
was one of the centers which made use of agate on an industrial scale.
Where in the beginning locally found agates were used to make all types
of objects for the European market, this became a globalized business
around the turn of the 20th century: Idar-Oberstein imported large
quantities of agate from Brazil, as ship's ballast. Making use of a
variety of proprietary chemical processes, they produced colored beads
that were sold around the globe.
[5] Agates have long been used in arts and crafts. The sanctuary of a Presbyterian church in
Yachats, Oregon, has six windows with panes made of agates collected from the local beaches.
[6]
A 15-pound tumbler barrel full of glistening tumble-polished agate and jasper.
See also
Notes
References
- The Nomenclature of Silica by Gilbert Hart, American Mineralogist, Volume 12, pages 383-395, 1927
- International Colored Gemstone Association
- Mindat data
- Meaning of various agate gemstone with images
- Schumann, Walter. Gemstones of the World. 3rd edition. New York: Sterling, 2006.
- 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.
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This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.
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