Agate is a cryptocrystalline 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.
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 sometime between the 4th and 3rd centuries BC. Colorful agates and other chalcedonies
were obtained over 3,000 years ago from the Achates River, now called Dirillo, in Sicily.
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.
Formation and characteristics
Most agates occur as nodules in volcanic rocks or ancient lavas, in former cavities produced
by volatiles in the original 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 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, 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.
When the matrix in which the agates are embedded disintegrates, 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 along 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. 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 and chunks of entrapped detritus. 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. Enhydro agate contains tiny inclusions of
water, sometimes with air bubbles.
Turritella agate is formed from silicified fossil Elimia tenera 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; Botswana agate; blue
lace agate; plume agate; condor agate, tube agate; fortification agate; fire agate; and
Mexican crazy-lace agate, which often exhibits a brightly colored, complexly banded pattern.
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. 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. Health impact
Respiratory diseases such as Silicosis and higher incidence of Tuberculosis among workers
involved in the agate industry has been reported from India and China.
See also List of minerals
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.
External links Media related to Agate at Wikimedia Commons