Agate is a type of quartz (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 but can be common in certain metamorphic rocks.[1]
The colorful agate and other chalcedonies were obtained over 3,000 years ago from the
Achates River, now the Dirillo, in Sicily.[2]
The stone given the name by a Greek by the name of Theophrastus, who discovered the stone along the shore line of the river
Achates sometime between the 4th and 3rd centuries BC.
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. 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 deposit may cause 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
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 67 tons was exhibited at the Dusseldorf 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.
Banded agate (agate-like
onyx). The specimen is 2.5 cm (1 inch) wide.
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 quartz, often amethyst, 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
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 (named varieties include Maury Mountain, Richardson Ranch, Sheep Creek and
others). Dendritic agates have beautiful fern like patterns on 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 fossil Turritella shells silicified in a chalcedony base.
Turritella are spiral marine 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 agate or stone.
California's "Mojave Blue" agate has gained a great deal of attention in the past several years. This pastel blue or blue-gray
agate cuts into attractive cabochons for jewelry and, in the hands of an expert carver, makes outstanding carvings.
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 fo making
jewelry and beads. Today any agate of this colour from Greek/Sicily area of the Mediterranean is called Greek agate. The stone
had been around centuries before that known to both the Summerians and the Eqyptians, who used the gem for decoration and
religious ceremony.
Another type of agate is Brazilian agate, which is found as sizeable 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 carnelian agate (usually exhibiting reddish hues), Botswana agate, Ellensburg blue agate, blue lace agate, plume agate (such as Carey, Graveyard Point, Sage, St. Johns, Teeter
Ranch and others), tube agate (with visible flow channels), fortification agate (which exhibit little or no layered structure),
fire agate (which seems to glow internally like an opal) and Mexican crazy-lace agate (which exhibits an often brightly colored,
complex banded pattern).
Agate beliefs
In many traditions agate is believed to cure the stings of scorpions and the bites of
snakes, soothe the mind, prevent contagion, still
thunder and lightning, promote eloquence, secure the favour of the powerful, and bring victory over enemies.[citation needed] Persian magi are also known to have prized agate rings in their work and beliefs.
Some followers of Pagan religions also believe agate is a crystal whose powers can be used for love, mental clarity, and good
luck in card games.
Shia Muslims often wear an agate ring on the right hand, the stone engraved with the name
of Allah, Ali, or the names of the other eleven Imams. It is known as "aqaq" or "agag" in Persian.
Uses in industry
Industry uses agates chiefly to make ornaments such as pins, brooches, paper knives, inkstands, and seals. 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.
See also
Notes
- ^ Donald W. Hyndman, David D. Alt (2002). Roadside Geology of Oregon, 18th, Missoula,
Montanat: Mountain Press Publishing Company, p. 286. ISBN 0-87842-063-0.
- ^ Agate Creek Agate. Retrieved on 2207-07-01.
References
Wikimedia Commons has media related to:
- The
Nomenclature of Silica by Gilbert Hart, American Mineralogist, Volume 12, pages 383-395, 1927
- International Colored
Gemstone Association
- Mindat data
- Schumann, Walter. Gemstones of the World. 3rd edition. New York: Sterling, 2006.
- http://www.gemstone.org/gem-by-gem/english/agate.html
- 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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