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Agate Gemstones - Chalcedony type of quartz variation

Introduction

Chalcedony is a cryptocrystalline type of quartz and agate falls into this classification of gemstones. This is a gemstone that is not formed as a single crystal, but as a cluster of several small crystals which are not visible to the naked eye, but can be clearly seen under high magnification. This is a gemstone that gets its name from the place where it was discovered, and this is at the River Achates, which is now known as River Dirillo, which is found in the southwestern parts of Sicily. Agate is well known for its striking bands which seem to flow along the stone. These patterns are not only seen on the surface of the stone but also in the interior. These bands are formed by the presence of many types of minerals within the stone, but the most common are iron and manganese. Agate is said to be one of the oldest gemstones in the world, and has been used by artists and lapidaries from all over the world, for centuries.

Gemstone aficionados agree that there is no other gemstone in the world that has the type of bands that you find in an Agate crystal. This is a quartz that forms in concentric circles, which brings about the formation of the textures and colors that are seen. Agate, is formed individually when water fills a cavity in the host rock, containing the minerals, and the n crystalizes with time. This is the reason why the rough stone is found as a round formation which has the concentric bands which look like the rings that you would find in a tree trunk when it is cut. There are times when the bands can resemble an eye, or some scallops, and sometimes even look like a landscape filled with trees.

In ancient times Agate was highly placed in society as an amulet or talisman. It was said to help heal fever, and also quench the thirst of the wearer. It is said that the Persian magicians could turn away storms using this gemstone. Mithridates, the King of Pontus, is said to have collected between 2 and 4 thousand agate bowls, and this shows how popular the gemstone was in ancient times. In the Byzantine Empire, agate bowls were used by the high and mighty and they were a symbol of status. European Royalty have also had the propensity to collect agate bowls during the Renaissance period. Many museums in Europe have collections of such bowls; one notable collection can be found at The Louvre.

The establishment of the famous cutting center known as Idar-Oberstein can be attributed to the finding and mining of agate in the Nahe River Valley in Germany; this is documented to have occurred around 1497. Initially, the waters of the river were used to move the huge grinding wheels of the cutting center. It is after the agate deposits on the Nahe had been exhausted, in the 19th century that the cutters at the center stared looking for agate from Brazil. There was a flurry of exploratory expeditions, and this led to the discovery of new deposits in Brazil, and the discovery of other gemstones such as tourmaline, topaz, amethyst and citrine among many others.

Idar-Oberstein is still know to be the finest agate cutting center in the world, but it does not get any agate from Nahe, and has to look for the mineral from other parts of the world. These are then cuts and set in Germany and Asia. Modern lapidaries and Vignette master carvers are doing well, as well as rough0stone dealers, who travel all over the world looking for the latest agate deposits available for export. This is an industry that owes its success to the way that people valued the agate bowls in the olden days; the renaissance is the one that can be attributed to the success of this gemstone. This is perhaps why the agate is said to bring success; it may have been a talisman unto itself.

Etymology and History of agate gemstones

A Greek philosopher and naturalist known as Theophrastus is the one who is credited with having discovered and named agate, when he stumbled upon it on the River Achate. This was said to have happened between the 4th and 3rd centuries. It is said that many colorful stones were obtained from over 3,000 years ago on the bed and banks of this river, which is found in Sicily, and is now called the Dirillo River.

The ancient use of this gemstone

One of the most common materials used in carving items from hard stone is agate. Archeologists have recovered several carvings from ancient sites that have been made using this rock, and the practice was widespread all over the world and not restricted to certain parts. You can consider the archeological find which is at the Knossos site on Crete Island, and the role that it played in the Bronze Age Minoan Culture.

The formation and characteristics of agate gemstones

Agate rough is found as nodules which form in ancient lava flows and other volcanic rocks. They are formed when volatiles in the original molten mass form cavities, in which the agates grow in banded nodules. This happens when silicate matter is deposited on the walls of the cavities, thereby forming a crystal that seem to grow inwards until it is filled completely. This stone is also known to grow in the veins and cracks of volcanic rocks; they are also found in altered rock which is underlain by intrusive masses of granite. The agate that is found in the cavities has banded rings and that which is found in veins and cracks has parallel lines running across the length of the rough stone. The bands and how they are formed will give rise to either Banded agate, Riband agate, or Striped agate.

Ordinary agate is formed by a very interesting way. Water that contains silicates, which have been dissolved from the surface, trickled, or percolated into the cavities of the volcanic or metamorphic rocks, and when it coats the walls of these pockets, it begins to crystallize. The siliceous material is deposited from the outside, towards the interior. The solutions do have a variation in the type and concentration of the silica material, and these differences are the ones that give the bands their different colors. This is why the bands of Chalcedony and those of crystalline quartz alternate with one another. When the rock is still in a viscous state, the vapor-vesicles may unite, and therefore form a very huge cavity and this may be the home of one of the largest agate rocks ever; such a process gave rise to the famed Brazilian geode, which was lined with amethyst, which weighed in at 35 tons, and was exhibited, in 1902, at the Düsseldorf Exhibition. Moxon, in a recent text on the review of the agate, gave a comprehensive description of the process.

According to Moxon the first deposit which lines the wall of the cavity is usually dark green in color, due to a mineral such as celadonite, delessite, or what is known as “green earth”, which are all known to have a high level of iron, which comes from the decomposition of the augtites found within the enclosing volcanic rock, and this layer is what is known as the skin of the agate resultant agate.

The green silicate matter will also give rise to a brown substance known as Limonite, which give the skin of agate a rusty appearance. When the agate is removed from the cavity, it has a lot of pits and is very rough, and this is because the original matrix is still left attached on the walls of the cavity. This original layer is also said to be the priming on which the rest of the stone is formed. The zeolitic materials are deposited with time, giving the agate the banded or striped appearance.

Most agates are hollow on the inside, since the deposition of the silicate matter was not enough to fill the insides of the stone, before the entrance to the stone was sealed. When this happens, the last deposit of the stone is known as the drusy quartz or amethystine, and it has the apices of the crystals looking towards the empty space, and they thus form a crystal lined geode or cavity. Usually these crystals are set free when the matrix disintegrates, thus releasing the agates. Agates are resistant to erosions, and they will remain as nodules in the soil; they can also be found on shores of rives and stream where they are deposited by the moving water.
The many types of agate

Mexican agate

This is the type of agate that is characterized as having a single eye and this has led it to be given the name of Cyclops agate. The matter which is included in the stone has red, golden, green, black and other color combination which are entrenched in the chalcedony. They are spread in filaments, and other forms which look like vegetable growths, and they give rise to dendritic or moss agate.

Dendritic agates are those which have a form that resembles ferns and these come about due to the presence of iron and manganese oxides. Other minerals which are added during the formation of the agate include radial mineral crystals which are known as sagenitic growths. You will also find detritus such as ash, mud and sand embedded in the stone. It is not uncommon to find agate material forming in vegetative material which has decomposed; these include tree trunks and roots, and these are called limb cast agate. Agate which has water and air bubbles is also called Enhydro agate.

Turitella agate

This is a type of agate, which is found in the silicified fossil of Elimia tenera shells, which were erroneously known as Turritella. These gastropods have elongated and spiral shells which have several whorls. It is also possible to find agate growing in petrified wood, porous rocks, and coral; any agate that is found in coral is also called Petoskey agate.

Greek agate

This is the form of agate which has a pale or tan color, and dates back to 400 BC and is commonly found in Sicily. It was commonly used by the Greeks to make beads and jewelry and hence the name. This is a stone that has been used for centuries by the Egyptians and Sumerians in the making of jewelry, beads and other items; despite this fact, the pale agate, even when it is found in other parts of the world, is still referred to as Greek agate.

Brazilian agate

This is a type of agate that is found in large geodes, having layered nodules. They are found to occur in brown tones which are layered with others of white and gray. There are quartz crystals which form within the nodules and these create a stunning specimen when they are cut in a manner that is opposite the axis of growth. For ornamental purposes, this agate is dyed in various colors.

Polyhedroid agate

This is the form of agate which has been formed in a flat shape that can be likened to a polyhedron. This is a stone that shows a unique formations of concentric polygon layers, when it is sliced open. This is a type of agate that is common to the Paraiba area of Brazil. This type of agate is not through normal crystallization processes, hut is said to be due to the filling up of the spaces that have been left after the matrix between pre-existing crystal has dissolved, and new material flows in and crystalizes.

Rainbow agate

When thin sections of certain agate are examined using transmitted light, they exhibit a diffraction spectrum, since the successive bands are very delicate. The spectrum resembles a rainbow and hence the name. This type of agate coexists with masses or layers of jasper, crystalline quartz and opal, and these occur due to certain varieties when the stones are being formed.

Apart from the above agate types, there are several others found in the world. The Botswana agate; Blue lace agate; tube agate, which has pinhole, or visible tubes in the structure; condor agate; carnelian agate, which has reddish tints; fortification agate, which has bands which remind the viewer of the ditches that were dug to protect troops during wars and also around forts; fire agate, which has a fire or internal flash, which id caused by a layer of white agate formed by hematite and the Mexican crazy-lace agate, which has a complex formation of brightly colored bands. The Mexican crazy-lace agate is also known as the Rosetta Stone or Rodeo agate, depending on the current owner of the mine at the time that it was found.

The uses of agate

The chief uses of agate are in the making of ornaments and other items such as pins, brooches, paper knives, marbles, seals, ink stands. Today, agate is used to make cabochon, beads, Intarsia art, carvings and decorative displays. They are also used to create face-polished and tumble-polished collector specimens which are classified according to their origin. Agate is very hard and can withstand the effect of acids and it is therefore used to make pestles and mortars which are used to mix various chemicals and corrosive substances. It has also been used to burnish leather for centuries due to the fact that it can be polished into a finely smooth surface.

Idar-Oberstein is the first and largest center where agate was used on an industrial level. Around the turn of the 29th century this center became a globalized business, even though it was originally built to be the center for the creation of agate items for the European markets. After the depletion of the stones in the area the center began to import the stone from Brazil in the form of ship’s ballast. The center then used proprietary processes to chemically produce items and beads which were exported to all parts of the world. The Presbyterian Church, in Yachats, Oregon, hoists great agate window panes, which were designed using agate collected from the local beach. This tells of how much the agate has been used for decorative and industrial purposes.

However, the mining of agate comes with a health hazard, and workers in some mines in India and China have been found to develop silicosis and tuberculosis.

Some interesting mythology of agate

If you lived in the Roman Empire, over 2000 years ago, and you took a walk on their magnificent paved roads, you would often meet with an ox-drawn cart, when a farmer was going to the field to harvest his crop. As you made way for the ox to pass, you would notice and agate amulet placed on the right horn of the ox, or the farmer would be wearing one on his right arm. If you asked, you would be told that this was a moss agate, which was known to give the wearer great harvests. It was an offering to the gods so they could make the harvest to become abundant. The moss agate has a look resembling dendritic inclusions of vegetation and trees. This is the reason why it was associated with the growth of crops. The moss agate was the best friend of a farmer in those days.

If you were going to join the Roman army, you would come across soldiers who would also be wearing quartz amulets. These were the type that has a brown or red color, and they were said to be protective against snakes and spiders, and that they also gave the soldiers a lot of courage when on the battlefield. The farmer that you met and the soldiers could also be wearing green quartz since it is said to better eyesight to the wearer.

In summary


Here are some simple points that you can easily memorize about the description and types of agates:

  • Single-color gems and patterned ones are also known as Chalcedony, which are made of tiny fibers and grains forming cryptocrystalline quartz.
  • The Chalcedony which have patterns, which are formed due to the deposition of mineral oxides mixed with silica, and accumulate over time are what you now call agate.
  • The agates that have a simple color banding pattern and called banded agates
  • The agates that have patterns which mimic natural landscapes and scenes are called scenic agates
  • The agates that have a lot of brilliant layers that seem to glow like embers in a fire are called fire agates
  • The agates that have banded swirls and swoops are called lace agates.
  • The agates that have patterns that resemble vegetation are called moss agates.
Recently in Namibia, one of the most-famous chalcedony was found, having a highly transparent slate-blue tint. Moss, Fire and banded agate still remain as the most popular among gemstone enthusiasts; this is because no two stones are alike, making each one to be a unique possession.



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