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White Chalcedony on Gray Chalcedony

The Mineral chalcedony




Chalcedony is not scientifically its own mineral species, but rather a form of Quartz in microcrystalline form. However, the name is an old name, and almost all mineral reference guides and collectors distinguish Chalcedony separately from Quartz. In the gem trade, the name Chalcedony usually describes only white or blue Chalcedony, to distinguish it from the multicolored banded variety Agate and other unique varieties of this mineral.

Chalcedony is quite varied in its formation habits. It sometimes occurs in geodes, lining the cavity with mammilary blobs. Its Agate variety is also found in geodes, commonly lining the outer layer underneath the larger Quartz crystals. Chalcedony also forms pseudomorphs after organic material. A well-known example is petrified wood, in which the wood has been completely transformed into Chalcedony. In the Petrified Forest National Monument in Arizona, an entire forest was transformed into petrified wood. Remains of this ancient forest can be seen in the huge silicafied logs that are found in the area.

Another well-known pseudomorph is Chalcedony after coral. In the Tampa Bay in Florida, coral has been chemically replaced by Chalcedony, and its original form is preserved. Another famous Chalcedony pseudomorph is Tiger's Eye. This popular variety has very unique optical properties in the form of a bronze sheen that is caused by the fibrous mineral Crocidolite that was chemically replaced into Chalcedony through pseudomorphism.

Impurities are frequently present in Chalcedony. They may form a moss like growth in the mineral, forming what is known as Moss Agate. Another example is Dendritic Agate, a variety of Chalcedony containing manganese oxide impurities that form fabrications resembling trees. These forms of Agate are not true Agates, since they lack the banding.

For additional information, see the gemstone section on Chalcedony.
Chemical Formula SiO2
Composition Silicon dioxide
Color White, blue, red, green, yellow, orange, brown, pink, purple, gray, black, colorless, and multicolored. Often banded in many different color combinations, and a few rarer forms are iridescent.
Streak White
Hardness 7
Crystal System Hexagonal
Crystal Forms
and Aggregates
Chalcedony, being a microcrystalline variety of the mineral Quartz, does not occur in visible crystals. It occurs in botryoidal, mammilary, stalactitic, massive, nodular forms, as smooth rounded pebbles, as banded masses, as amygdules, and as the linings of geodes.
Transparency Transparent to opaque
Specific Gravity 2.6 - 2.7
Luster Vitreous, waxy, or dull
Cleavage None
Fracture Conchoidal
Tenacity Brittle
Other ID Marks 1) Commonly fluorescent, usually green or white.
2) Triboluminescent
3) Piezoelectric
Complex Tests Dissolves in hydrofluoric acid
In Group Silicates; Tectosilicates; Silica Group
Striking Features Hardness and form
Environment Occurs in all mineral environments, especially in igneous environments.
Rock Type Igneous, Sedimentary, Metamorphic
Popularity (1-4) 1
Prevalence (1-3) 1
Demand (1-3) 1

Chalcedony ON EBAY
OTHER NAMES
Chalcedon
Chalcedonite
Chalcedonyx
Microcrystalline Quartz Term used for Chalcedony, microcrystalline form of Quartz, to distinguish it from crystalline Quartz.

VARIETIES
Varieties for Agate and Jasper are listed separately.
 -  Opaque multicolored Jasper, or Jasper with banding; may also refer to a single stone with a combination of both Agate and Jasper.
 -  Banded variety of Chalcedony. For additional information, see the mineral page on Agate.
 -  Opaque form of compact Quartz or Chalcedony containing small Mica, Hematite, or Goethite scales which cause a glistening effect. Although technically Aventurine is classified as rock due to its composition of several minerals, it most often is regarded as a variety of Quartz or Chalcedony. For additional information, see the gemstone page on Aventurine.
 -  Chalcedony with dense, parallel inclusions of Goethite. Its color is a multicolored yellowish to reddish, and it exhibits chatoyancy. Binghamite was found in the Cuyuna Iron Range in Minnesota, and is used as a rare gemstone.
 -  Dark green to greenish blue variety of Chalcedony speckled with red or brown spots. (May also refer to Hematite with red or brown spots.) For additional information, see the gemstone page on Bloodstone.
 -  Red to amber-red translucent variety of Chalcedony. For additional information, see the gemstone page on Carnelian.
 -  Compact form of Chalcedony typically found as nodules in sedimentary rocks, usually with an off-white, gray, or cream color. Although technically Chert is classified as a rock due to its composition of other mineral impurities, it most often is regarded as a variety of Chalcedony.
 -  Apple green variety of Chalcedony. For additional information, see the gemstone page on Chrysoprase.
 -  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.
 -  Massive, uniformly colored form of Chalcedony that is somewhat impure. Flint is well-known for its sharp edges and usage as tools such as arrowheads by the early Native Americans. Although technically Flint is classified as a rock due to its composition of other mineral impurities, it most often is regarded as a variety of Chalcedony.
 -  Pseudomorph of compact Quartz after the fibrous mineral Crocidolite, with a chatoyant sheen, very similar to Tiger's Eye. It has a bluish-gray color, which differentiates it from Tiger's Eye which has a yellow-brown color. It is formed from an incomplete pseudomorph process.
 -  Translucent, violet-blue variety of Chalcedony, often used as a gemstone. It is sometimes banded.
 -  Opaque form of Chalcedony, most often used to describe brown, yellow, or reddish colors. For additional information, see the mineral page on Jasper.
 -  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.
 -  Agate/Chalcedony or Opal with red bands or red spots of the mineral Cinnabar. It is named after Myrick Spring, San Bernardino Co., California.
 -  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.
 -  Petrified Wood is wood that chemically replaced by a mineral substance. The replacement is usually Chalcedony, but Opal and a few other minerals are also known to replace the wood. When the wood becomes petrified, its original mold remains intact, but an entire new substance takes the place of what was once wood. When the mineral replacement of the wood is Chalcedony or Opal, the substance is also called "Silicafied Wood".
 -  Dark green variety of Chalcedony. It often contains small white or yellow spots.
 -  Light to emerald green, transparent to translucent Quartz, with coloring caused from inclusions of green minerals, such as Actinolite, Hedenbergite, Chlorite, or Malachite.
 -  Brownish to brownish-red, transparent to translucent form of Chalcedony.
 -  Form of Agate with parallel bands of brownish to red alternating with white or sometimes black bands.
 -  Pseudomorph of compact Quartz after the fibrous mineral Crocidolite. Tiger's Eye is famous for its chatoyant effect. For additional information, see the gemstone page on Tiger's Eye.

USES
Chalcedony is a very important ornamental stone. The varieties Agate, Chrysoprase, Carnelian, Sard, Tiger's Eye, Bloodstone, Jasper, and Moss Agate are all carved into cabochons and beads, making fine yet inexpensive gems. The apple-green variety, Chrysoprase, has a distinct color and commands a higher price than the other varieties. Light blue Chalcedony has also recently become popular as a gemstone.

The Chalcedony varieties are very popular among amateur collectors and sold in tourist shops worldwide, especially in tumbled form.

NOTEWORTHY LOCALITIES
Chalcedony is extremely common and found worldwide in abundance. The localities of the banded variety Agate will be mentioned on a separate page. The original source of Chrysoprase is Szklary, Ząbkowice, Poland; and significant deposits are in Marlborough, Queensland, Australia. The only significant deposits of Tiger's Eye are in Northern Cape Province, South Africa. Blue Chalcedony comes from the Chikwawa District, Malawi; and from Blinkpan, Namibia. Nice Chalcedony also comes from the Deccan Basalts of India, such as in Nasik and Jalgaon.

Excellent Chalcedony pseudomorphs after coral come from several places in western Florida; notably the Tampa Bay, Hillsborough Co.; Tarpon Springs, Pinellas Co.; south of New Port Richey, Pasco Co; and along the banks of the Suwanee River in Hamilton, Columbia, and Suwanee Counties.

COMMON MINERAL ASSOCIATIONS
Quartz, Calcite

DISTINGUISHING SIMILAR MINERALS
Few minerals can be distinguished from Chalcedony. Opal may have similar forms, but its lower hardness can distinguish it.


chalcedony PHOTOS
Images for the varieties Agate and Jasper are listed separately.
 
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