Agate is a variety of microcrystalline quartz that displays translucence and in some cases banding. Agate primarily forms when silica-rich fluids fill pockets within rock and/or fossils, resulting in deposition of the silica along the walls of the rock. This process can result in banding patterns as the composition and impurities of the fluids change over time. These banding patterns can either form as flat layers, or rounded layers depending on the surfaces available for deposition.
Agate is a type of chalcedony (variety of quartz) that forms when a silica-rich solution fills an open cavity within rock and begins to deposit crystals evenly along the walls. This deposition occurs molecule by molecule, forming tightly packed fibrous microcrystalline quartz crystals that line the contact walls of the cavity. Depending on impurities present during formation, the agate will exhibit varied coloration as the impurities work their way into the crystal lattice. In some cases this can result in concentric banding within a nodule, or layered formations within cracks (seams, vugs, veins, etc.) between rocks.
Agate within Nodules or Geodes - Nodular or geodic agate formations occur in bubbles within the host rock that were created during volcanic activity. These bubbles within the rock formed as a result of rapid gas formation from expansion of volatile components. They moved their way up through the rock as it was still in a viscous state. Eventually the gas escaped through the rock following the rock solidifying and cracking. Mineral-rich water then made its way into the hollow cavity and provided the necessary minerals were present, agate could then form within the cavity. These agate formations occur throughout the world in locations where volcanic activity has created these environments, explaining why agate nodules and geodes can be found on every continent.
Agate within Seams or Vugs - Cracks occur within rock formation for a variety of reasons (tectonic plate movement, landslides, earthquakes, etc.). Water can then permeate through these cracks through both ascending (hydrothermal) and descending fluids. If the water is mineral rich and/or has picked up minerals from the surrounding host rock, crystal formations can occur, in this case agate.
Types Of Agate.
Wall-Lining Banded Agate - A very common form of agate. This is the name for agate that deposits along the walls of a cavity. The microscopic crystals deposit in layers that can vary in color dependent on the impurities present during formation. As the space fills, the layers of agate follow the shape of the surrounding rock, in some cases giving the specimen unique shapes as the layers shrink. This type of formation can occur in both spherical cavities or when cracks in the host rock are filled with a silica-rich fluid (seam agate).
Horizontal Banded Agate - A less common form of agate banding where the agate deposits in horizontal layers. These layers often occur within cavities that have already been partially lined with agate. What's special about these agate formations is the ability to visualize the vertical orientation of the geode during agate formation. In some cases, the specific orientation of the geode can be seen in geodes that contain horizontal banding and a cavity.
Agate Geode - A geode cavity within an agate nodule will occur if the silica solution runs out before the entire cavity can become filled. In most cases, this will result in quartz crystals that line this cavity. These crystals can range anywhere from druzy sized all the way up to massive crystals depending on the size of the cavity.
Fortification Agate - Like banded agate, it features layered agate formations that follow the general shape of the host rock. What's different about this type of agate is the sharp angles of the bands as well as the small projections that come off of these angles, resembling an overhead view of a castle wall and its fortifications.
Dendritic Agate - Agate that contains dendrite formations resembling tree branches, ferns or lightning patterns. These are a result of iron or manganese ions that seep into small fractures throughout the agate. The ions then crystalize, leaving behind these branching patterns.
Sagenite Agate - This specific formation occurs when agate fills a cavity that previously contained acicular (needle-like) sagenite (rutile) crystals. This similar type of formation can occur with all sorts of minerals (aragonite, stibnite, epidote, tourmaline, etc.). In these cases the agate would be considered a pseudomorph. The mineral inclusion within the agate can also determine the name that the agate formation is classified by.
Iris Agate - A formation of agate that features thin bands of agate that often form in the typical concentric banding pattern. When cut properly and provided with light at a specific angle, this type of agate will exhibit a colorful rainbow effect throughout the bands. Iris agates that produces the best color effects are typically colorless.
There are countless agate formations around the world, many of which have been given their own individual names. These names often come from dealers or collectors who use these names to help classify them based on their appearance or extraction location. The types of agate that are listed above are common formations whose names were designated by scientists and/or were widely accepted over time.
Agate Or Chalcedony?
Agate is a variety of chalcedony. Chalcedony is the classification for minerals that are composed of microcrystalline quartz (silicon dioxide - Si02). When chalcedony is translucent and/or banded, it is typically given the name agate. An example of a similar situation is jasper. Jasper is also a variety of chalcedony, meaning it's primarily composed of microcrystalline quartz, however unlike agate, jasper is opaque.
Microcrystalline or Cryptocrystalline? - The term microcrystalline is near synonymous with cryptocrystalline, meaning they can be used interchangeably to describe a microscopic sized crystal.
Formations Of Agate By Locale.
Mexico - Bird of Paradise Agate - Bird of Paradise Agate is found near Naica, Chihuahua, Mexico, and often features Marcasite inclusions along with many colors of plumes. The beautiful plumes or dendrites that occur within rock are most often a result of intruding (percolating) mineral solutions that contained manganese and/or iron. These solutions can work their way through microscopic cracks in the rock, leaving behind a branching pattern throughout the rock. By an untrained eye, these can be mistaken for plant fossils within the rock.
Brazil - Brazilan Agate - Agates from Brazil are mined from decomposed volcanic ash and basalt from the late Permian age (248-275 million years old) around Rio Grande do Sul. These agates can be very colorful and polish beautifully.
Oregon - Carey Ranch Plume Agate - Carey Ranch Plume Agate is an agate formation that was mined from the Carey family ranch near Prineville, Oregon. The first documented find of this agate was a small find that had weathered out from a small volcanic cone. Following some exploring, the outcrop of this agate formation was found and the mining process began. After years of dirt and rock removal from this volcanic cone, only a large pit remains in its place. This location now produces very little, if any of these beautiful plume agate specimens. Consequently, Carey Ranch Plume Agate is now considered "extinct" and the only specimens found on the market are from old collections.
Argentina - Condor Agate - Condor agate is a highly-prized gem and mineral specimen from the Mendoza Province of Argentina. Most specimens feature red, brown, and black banding, but colors can vary into blues, greens, and even white. A relatively new discovery, only known outside of Argentina since the early 1990s, Condor agate makes a wonderful addition to any mineral collection.
Mexico - Crazy Lace Agate - Crazy lace agate, also known as Mexican Agate is a unique banded agate that comes from Chihuahua Mexico. It gets its name from complex, lacy patterns swirling through the stone. Colors can vary from creme to orange, to red colorations and are due to iron and aluminum inclusions in the agate.
Madagascar - Flower Agate - Flower agate is a formation of agate that contains plumes of white-pink, opaque chalcedony formations that are scattered throughout. Occasionally these specimens are found with white and pink banded agate, as well as milky white cross sections of quartz crystals. This type of agate comes out of Madagascar and is most often sold as a polished stone. Some of these polished stones contain pockets of botryoidal agate formations that resemble large spherical grape agates.
Oregon - Graveyard Plume Agate - Long known for its beautiful "plume" inclusions that look like moss or snow-covered trees, agate from this area comes from just a couple of small mines. Plumes or dendrites that occur within rock are most often a result of intruding (percolating) mineral solutions that contained manganese and/or iron. These solutions can work their way through microscopic cracks in the rock, leaving behind a branching pattern throughout the rock. By an untrained eye, these can be mistaken for plant fossils within the rock.
Minnesota - Binghamite Agate - Binghamite, also known as "silkstone" or "cuyunite", is a highly sought after variety of agate that is only known to come from one location in Minnesota. This agate formation is found near the Cuyuna iron range in Crow Wing County, Minnesota. It often displays vibrant colors of red, orange and yellow and in some cases contains small veins of iron running throughout the agate. Many specimens have chatoyant properties that are similar to tiger's eye. The chatoyance can be attributed to the fibrous hematite and goethite parallel crystal formations that the agate deposits around.
Mexico - Laguna Agate - Laguna agate is a highly sought after variety of banded chalcedony. It often forms in bands of red, pink, white and/or scarlet hues, with some specimens containing bands of brown and golden yellow chalcedony. The first documented finds of this agate occurred in the late 1940's and since then have been extracted from several sites near Ojo Laguna, Chihuahua, Mexico. These agates are believed to be roughly 38 million years old.
Turkey - Turkish Stick Agate - Stick Agate is a formation of agate that comes from mines in Ankara, Turkey. Much of the time, this agate features stick-like formations that are surrounded by agate. This occurs when agate fills a cavity within a rock that already contains mineral formations throughout. In the case of these stick agates from Turkey, the agate formed as a pseudomorph after aragonite crystals. In some cases, these aragonite crystals form as hollow tubes that when polished will contain agate and/or druzy quartz within.
Oregon - Trent Stibnite Agate - Trent stibnite agate, also known as "Trent agate", is a very unique and rare agate formation which occurred around sprays of stibnite, native arsenic and/or realgar. Trent agate was collected in the 1960's, from a railroad cut in Trent, Oregon. Following an accident at the site of collection, the mine was closed and eventually covered by a road. Specimens of this beautiful agate are now only obtainable through old collections.
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