Listing description
Agate (pronounced /ˈæɡət/)
is a microcrystalline variety 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]
Detailed description
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. 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 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. 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
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 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. Today any agate of this color
from Sicily, once an ancient Greek colony, is called Greek agate. Yet the stone
had been around centuries before that and was known to both the Sumerians and
the Egyptians, who used the gem for decoration and religious ceremony.
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 (usually exhibiting reddish hues), Botswana agate, Ellensburg
blue agate, blue lace agate, plume agates, 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) also
called Rodeo Agate and Rosetta Stone depending on who owned the mine at the
time.
Uses in industry and art
Industry uses
agates chiefly to make ornaments such as pins, brooches, paper knives,
inkstands, marbles 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. 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]
Chalcedony has a
waxy luster, and may be semitransparent or translucent. It can assume a wide
range of colors, but those most commonly seen are white to gray, grayish-blue
or a shade of brown ranging from pale to nearly black.
The name
"chalcedony" comes from Latin calcedonius, the word used to
translate the Greek word khalkedon, found only once, in the Book
of Revelation;
according to the OED a connection with the town of Chalcedon in Asia Minor is "very doubtful".[3] There is no reason to assume that the
precious stone referred to by this name in the Bible is the same as what is now
understood by the name.[4]
Varieties
Chalcedony occurs
in a wide range of varieties. Many semi-precious gemstones are in fact forms of chalcedony. The
more notable varieties of chalcedony are as follows:
Agate
Agate is a variety of chalcedony with
multi-colored curved or angular banding. Fire agate shows iridescent phenomena
on a brown background; iris agate shows exceptional iridescence when light
(especially pinpointed light) is shone through the stone. Landscape agate is
chalcedony with a number of different mineral impurities making the stone
resemble landscapes.[5]
Carnelian
Carnelian (also spelled cornelian) is a
clear-to-translucent reddish-brown variety of chalcedony. Its hue may vary from
a pale orange, to an intense almost-black coloration. Similar to carnelian is sard,
which is brown rather than red.
Chrysoprase
Chrysoprase (also spelled chrysophrase) is
a green variety of chalcedony, which has been colored by nickel
oxide. (The darker
varieties of chrysoprase are also referred to as prase. However, the
term prase is also used to describe green quartz, and to a certain extent is a
color-descriptor, rather than a rigorously defined mineral variety.)
Moss agate
Moss
agate (also known as tree
agate or mocha stone) contains green filament-like inclusions,
giving it the superficial appearance of moss or blue cheese. It is not a true form of agate, as
it lacks agate's defining feature of concentric banding.
Onyx
Onyx is a variant of agate with black and
white banding. Similarly, agate with brown, orange, red and white banding is
known as sardonyx.
History
As early as the Bronze Age chalcedony was in use in the Mediterranean region; for example, on Minoan Crete at the Palace of Knossos, chalcedony seals have been recovered dating to circa
1800 BC.[6] People living along the Central Asian trade routes used various forms of
chalcedony, including carnelian, to carve intaglios, ring bezels (the upper faceted
portion of a gem projecting from the ring setting), and beads that show strong
Graeco-Roman influence.
Price
$249475.6/kg or $113398/ib
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