Agate is a gemstone that is used in various decoration pieces and jewelry. It is made of chalcedony and silica. The crystals of silica, mainly chalcedony form a grainy stone in bright colors. These crystals are known as Agate. The name “Agate” has been given to it after the river “Achates” where once it was found for the first time. Achates is located in southwestern Sicily, Italy.
It is mostly found in volcanic rocks or lavas. Agate has great power to fill the cracks in volcanic rocks once the lava outbursts from them. When agate is cut transversely, it shows a network of parallel lines. These lines appear mildly on the surface and seem to divide agate into different sections. Agate with such lines is known as banded agate.
Agate’s composition is usually the same. It is classified based on color and design. The main types of agate are listed below.
Onyx: Onyx consists of moganite and silica minerals quartz. It is cryptocrystalline with a prominent shape and parallel crystals. Banded agate has chaotic banding. Onyx is available in various colors like purple, blue, yellow, white, and many others.
Sardonyx: It has red and white bands, making it more elegant and prominent. The bands are parallel and similar to onyx.
Iris Agate: Iris agate has rainbow colors. Please note that it has no resemblance to the human eye, and the name is due to its color. It makes a beautiful reflection of light when placed against the light.
Crazy Lace Agate: It is also known as earth rainbow due to its vibrant and bright colors. It has banded chalcedony in white with other layers usually in blue, brown, cream, or black color. Some rate specimens also have yellow, orange, golden, and red colored bands.
Thunder-egg Agate: The thunder-egg agate is a nodule-like stone, usually equal to baseball but also available in smaller sizes. These agates are spherical and typically have a rough surface. Thunder-egg agates are usually found in rhyolitic volcanic ashes.
Enhydro Agate: These are also nodule-like stones with water trapped inside. The chalcedony agates are usually found in volcanic rocks. The agates are similar to fluid-inclusion agates.
Polyhedroid Agate: It has flat sides and in color-scheme, similar to Polyhedron. When it is cut from the center, the layers of concentric polygons can be seen inside. It is usually found in Paraiba State, Brazil. The crystals inside reflect the light and discharge beautiful rays when kept against the light.
Moss Agate: It is made of silicon dioxide, chalcedony, and green minerals. It usually has green and white filaments that can be seen when the stone is cut into two sections.
The Lake Superior Agate: It is commonly found on the shores of Lake Superior, North America, hence the name. It is red due to iron. The Lake Superior Agate is commonly available in Minnesota.
Condor Agate: It was discovered by Luis de Los Santos in 1993. It is an attractive and elegant agate due to its orange-colored patterns.
Sagenite Agate: It is formed with the combination of Chalcedony and different minerals. The golden colored hair-like pointed lines make its design more attractive and unique.
Fortification Agate: The name was given to it due to its sharp-edged bands. These bands resemble with a fortified castle with sharp angles. The array of white, light brown, orange, red and purple colors make it worthwhile.
Fairburn Agate: It is a rare and beautiful type of agate found in Fairburn, Custer Co., South Dakota, USA. It is considered the state gem of South Dakota, USA. The stone is found in the volcanic rocks. You can see its patterns and designs by cutting the rock. The surface is rough when found naturally.
Botswana Agate: It is a unique agate available in dark and light gray and pink shades. However, some layers come with muted brown color.
Dendritic Agate: It is called the Stone of Plentitude. It is considered the most precious type of agate. Dendritic agate is related to the ancient Greek dryads. So, this type of agate is the sign of good luck and the farmers use to bury it into their fields to get the good crops.
Coyamito Agate Pseudo-morphs: Coyamito is also considered a unique form of agate that is not easily available. It is found in only a few locations in the world.
Turritella Agate: It is formed from shells of fossilized freshwater Turritella gastropods. Agates can also form from coral, porous rocks, petrified wood, and other organic remains can also form agates.
Coldwater Agates: These agates formed within the limestone and dolomite rocks of marine origin. Coldwater agates also form from silica gels. These agates are usually less colorful and have banded white and grey chalcedony lines. Coldwater agates are found in Lake Michigan.
Greek Agate: It is a pale white to tan colored agate. Its reserves are located in the Greek colony of Sicily, Italy. Greeks used it to make jewelry and beads.
Brazilian Agate: These agates are found as sizable geodes of layered nodules. The agates have a brownish tone that is interlayered with white and gray. Brazilian agate is dyed and processed for jewelry making.
Holley Blue Agate: It is rare dark blue ribbon agate found only near Holley, Oregon.
Binghamite: An agate variety found only on the Cuyuna iron range (near Crosby) in Minnesota.
Fire Agate: It is named after its appearance. The agate shows an iridescent internal flash that looks like fire. Its appearance results from clear agate and hematite (hydrothermally deposited) layers.
Patuxent River Stone: A type of agate found only in Maryland.
Agate usually forms on or within pre-existing rocks. It is usually found in igneous or metamorphic rocks. Some sedimentary rocks like limestone or dolomite also contain agate.
Since agate is found inside other rocks, it is difficult to calculate its age. However, it is usually found in rocks dating back to Archean Eon (4000 – 2500 million years ago). Agates are usually found as nodules inside volcanic rock cavities. The cavities form when gasses are trapped within the magma. These cavities are later filled with silica-rich fluids, resulting in layer creation. The first layer is called the priming layer.
The variations in magma composition and deposition conditions dominate the type of agate that will form in the rocks. For example, the variations form banded agate when chalcedony is found in alternating layers of quartz. Hollow agates form when liquid-rich silica deposition is not deep enough to fill the igneous cavity completely.