The Mineral agate
Agate is the banded form of the mineral Chalcedony, which is a microcrystalline variety of Quartz. Agate is the most varied and popular type of Chalcedony, having many varieties on its own. Although the pattern on every Agate is unique, the locality of an Agate will provide resemblances in banding style and color, thus lending many Agates with a geographic prefix. Some examples are Laguna Agate (named after Ojo Laguna, Mexico) or Botswana Agate (after the African country of Botswana). Other variety names used connote specific colors or patterns, such as Fire Agate or Eye Agate.
Agate usually forms in rounded nodules or knobs which need to be sliced open to bring out the internal pattern hidden in the stone. Most Agate is ugly in its natural state; specimens must be polished to bring out their full beauty. Much of the Agate sold to collectors has been treated, in the form of tumbled stones or polished slabs. Popular collector forms of Agate include nodules or geodes sliced in the middle into two polished cross-sections, or thin slabs from nodule or geode cross-sections.
The formation of Agate is most often from deposition of layers of silica filling voids in volcanic vesicles or other cavities. The layers form in stages with some of new layers providing an alternating color. Since the cavities are irregularly and uniquely shaped, each Agate forms its own pattern based on the original cavity shape. When a cavity is completely filled, it forms a solid mass of Agate, but often it is only partially filled, leaving a hollow void which often has crystalline Quartz growths on its innermost layer. This is the cause of Agate forming the outer lining of most geodes.
Agate is often dyed to enhance its colors. This is
especially true of Agate from Brazil. Bright neon colors such as bright blue
and red are rarely natural.
Agate is named after the Achates River (now known as the Dirillo River) on the island of Sicily, Italy, whose upper waters were an ancient source of this gemstone.
For additional information, see the gemstone section on Agate.
Multicolored in banded formation. Colors include white, blue, red, green, yellow, orange, brown, pink, purple, gray, and black. Some rarer forms of Agate are iridescent.
There are many variety names of Agate that are generally used by
collectors and dealers, but there is also an abundance of variety names
that are made up by dealers to describe a locality or other habit. The
varieties section below lists only variety names that are
commonly encountered. An x before a variety name indicates that the name is generally not as accepted as the varieties that are checked below. One other note regarding Agate varieties is that one Agate may be legitimately classified under more than one variety. For example, an Agate may be both a Laguna Agate as well as a Fortification Agate.
Thick layer of Agate surrounding a cavity in a geode that is usually lined with a layer of small Quartz crystals.
Opaque multicolored Jasper, or Jasper with banding; may also refer to a single stone with a combination of both Agate and Jasper.
Agate from the Mexican locality of Agua Nueva. Agua Nueva Agate is known for its purple and pink banding formations.
Agate with light blue bands in a lacy or wavy pattern.
Agate from the African country of Botswana banded with fine parallel lines of white, purple, or peach.
Agate with broken fragments naturally cemented together; appears similar to breccia.
Agate from San Rafael, Argentina, often with bright colors.
Agate from Rancho Coyamito, Mexico, that often has reddish banding.
Agate with twisting and turning bands of various colors.
Translucent Chalcedony with tree-like or fern-like inclusions. Dendritic Agate is technically not a true Agate, as it lacks the banding patterns exhibited in Agates.
Agate nodule containing trapped water bubbles. The water can be seen from the outside of the nodule when held up to the light. Also known as Enhydritic Agate.
Agate with a pattern in which all bands connect to each other causing it to resemble a medieval fortress (i.e. imaginary moat and walls surrounding the castle).
Rare iridescent Agate that exhibits spectral colors on a translucent colorless or white base.
Well known form of colorful Agate with very dense banding from Ojo Laguna, Chihuahua, Mexico.
Agate from the basalt region of northern Michigan, near the shores of Lake Superior.
Agate that resembles a scenic landscape such as mountain formations.
Agate consisting of thin bands in a lacy or wavy pattern.
Agate from Estacion Moctezuma, Mexico, known for pastel colors.
Agate with a light pastel blue or blue-gray color from the Mojave Desert in California.
White to cream
Agate or Chalcedony with a wrinkled or cracked
"skin", resembling the skin of a snake; found in Oregon.
Distinct form of Agate from Agate Creek in Queensland, Australia.
Agate with acicular or or pointed inclusions of various minerals. These hair like formations are often arranged in fans or bursts.
Form of Agate with parallel bands of brownish to red alternating with white or sometimes black bands.
Agate with a scale-like layer that resembles the skin of a snake. Also refers to a reddish brown Agate with small black concentric bands.
Agate with star-shaped patterns of manganese oxide inclusions, found in the Sweetwater River, Wyoming. Sweetwater Agate is not true Agate as it lacks the banding patterns of
Agate, but is a form of Moss Agate.
Rounded nodule filled with Agate in the center. The term Thunder Egg is usually reserved for such nodules found in Oregon, but the term may also encompass similar nodules from other locations.
Agate with tube-like formations which are sometimes hollow.
Agate is carved into cameos and ornamental objects, and is very popular as polished slabs.
It is used as an inexpensive gemstone in jewelry, most commonly as beads in necklaces, bracelets, earrings, and cameos. Agate also makes beautiful and ornate bookends and carved figures.
Also see the gemstone section on Agate for additional information.
The first worked Agate deposits were near Idar-Oberstien, Germany, which has long since been commercially worked out. One of the most abundant sourcs of Agate today is the Rio Grande do Sul in Brazil, where many different sizes and types occur. These Agates are sometimes dyed. Other outstanding South American localities are San Rafael, Mendoza Province, Argentina; and Artigas, Uruguay. Mexico has several famous Agate sources in Chihuahua, the most important being Ojo Laguna, Moctezuma, and Rancho Coyamito. A well-known Agate deposit in Australia is Agate Creek, in Queensland. Poland has some good Agate finds in the Kaczawski Mountains at Nowy Kosciol and Ploczki Gorne.
In the U.S., fine Agate comes from the Dryhead Agate Mine, Carbon Co., Montana; Fairburn, Custer Co., South Dakota; Baker ranch, Luna Co., New Mexico; and from the shores of Lake Superior in the Keweenaw Peninsula of Michigan. "Thunder Egg" Agates are abundant from Jefferson Co., Oregon. The finest iridescent Fire Agate comes from the Black Hills, Graham County, Arizona; and in Mexico in Aguascalientes.
Common Mineral Associations
Quartz, Calcite, Goethite, Hematite, Celadonite, Chamosite
Distingushing Similar Minerals
The unique banding patterns and hardness will distinguish Agate from all minerals.
Since most forms of Agate are dull and uninteresting until they are cut and polished, we will show images mostly from the cut and polished forms for visual clarity.