Agate chalcedony: The mineral Agate information and pictures

About

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.

Crystal Forms & Aggregates

Agate is a banded microcrystalline form of the mineral Quartz, and does not occur in visible crystals. It occurs in nodules, in massive form, as botryoidal, mammilary, and stalactitic formations, as smooth rounded pebbles, as amygdules, and as the linings of geodes.

Other ID Marks

1) Commonly fluorescent, usually green or white. May even show fluorescent banding patterns where some of the bands will fluoresce more strongly than others.
2) Triboluminescent
3) Piezoelectric

Complex Tests

Dissolves in hydrofluoric acid.

Striking Features

Banding patterns

Environment

Agate occurs in all mineral environments, but it is most prevalent in igneous rocks such as basalt.

Varieties

✓ common · ✗ uncommon

Blue Lace Agate— Agate with light blue bands in a lacy or wavy pattern.
Botswana Agate— Agate from the African country of Botswana banded with fine parallel lines of white, purple, or peach.
Brecciated Agate— Agate with broken fragments naturally cemented together; appears similar to breccia.
Condor Agate— Agate from San Rafael, Argentina, often with bright colors.
Crazy Lace Agate— Agate with twisting and turning bands of various colors.
Dendritic Agate— 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.
Eye Agate— Agate with banded, concentric rings that are perfectly rounded.
Fire Agate— Form of Agate or Chalcedony that is iridescent with a play of colors or "fire" similar to that of Opal. Fire Agates usually have botryoidal bubbles included in their interior. The play of color is caused by inclusions of Goethite or Limonite.
Fortification 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).
Iris Agate— Rare iridescent Agate that exhibits spectral colors on a translucent colorless or white base.
Laguna Agate— Well known form of colorful Agate with very dense banding from Ojo Laguna, Chihuahua, Mexico.
Moss Agate— Chalcedony containing dense inclusions of green Hornblende that cause the pattern to resemble moss. Moss Agate is not true Agate as it lacks the banding patterns of Agate.
Onyx— Form of Chalcedony with a solid black color or white and black banding. Occasionally also refers to banded Travertine or Tufa in the mineral form of Calcite or Aragonite with black and white bands. For additional information, see the gemstone page on Onyx.
Sagenite Agate— Agate with acicular or or pointed inclusions of various minerals. These hair like formations are often arranged in fans or bursts.
Sardonyx— Form of Agate with parallel bands of brownish to red alternating with white or sometimes black bands.
Snakeskin Agate— 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.
Thunder Egg— 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 Geode— Thick layer of Agate surrounding a cavity in a geode that is usually lined with a layer of small Quartz crystals.
Agate Jasper— Opaque multicolored Jasper, or Jasper with banding; may also refer to a single stone with a combination of both Agate and Jasper.
Agatized Wood— Petrified Wood in the form of Agate, with banding patterns.
Agua Nueva Agate— Agate from the Mexican locality of Agua Nueva. Agua Nueva Agate is known for its purple and pink banding formations.
Cloud Agate— Grayish Agate with blurry, foggy patches of inclusions.
Coyamito Agate— Agate from Rancho Coyamito, Mexico, that often has reddish banding.
Dryhead Agate— Agate from Montana with orange and brownish banding.
Enhydro Agate— 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.
Fairburn Agate— Form of Fortification Agate from Fairburn, South Dakota.
Fossil Agate— Agate that forms as a replacement of organic material such as wood and shells.
Grape Agate— Spherules of Agate or Chalcedony clustered together in a botryoidal, grape-like habit.
Lake Superior Agate— Agate from the basalt region of northern Michigan, near the shores of Lake Superior.
Landscape Agate— Agate that resembles a scenic landscape such as mountain formations.
Mexican Lace Agate— Agate consisting of thin bands in a lacy or wavy pattern.
Moctezuma Agate— Agate from Estacion Moctezuma, Mexico, known for pastel colors.
Mojave Blue Agate— Agate with a light pastel blue or blue-gray color from the Mojave Desert in California.
Nipomo Agate— Agate with Marcasite inclusions found in Nipomo, San Luis Obispo Co., California.
Oregon Snakeskin Agate— White to cream Agate or Chalcedony with a wrinkled or cracked "skin", resembling the skin of a snake; found in Oregon.
Plume Agate— Agate with inclusions in feather-like patterns.
Queensland Agate— Distinct form of Agate from Agate Creek in Queensland, Australia.
Rainbow Agate— Iridescent Agate that exhibits a multicolored effect thin slabs.
Scenic Agate— Synonym of Landscape Agate
Sweetwater Agate— 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.
Tube Agate— Agate with tube-like formations which are sometimes hollow.

Uses

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.

Noteworthy Localities

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.