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(kălsĕd`ənē) [from ChalcedonChalcedon
, ancient Greek city of Asia Minor, on the Bosporus. It was founded by Megara on the shore opposite Byzantium in 685 B.C. Taken by the Persians and recovered by the Greeks, it was later a possession of the kings of Bithynia, from whom it passed (A.D. 74) to Rome.
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], form of quartz the crystals of which are so minute that its crystalline structure cannot be seen except with the aid of a microscope. Chalcedony has a waxy luster and is translucent to transparent. The name chalcedony is applied more specifically to white, gray, blue, and brown varieties. Some varieties, differing in color because of the presence of impurities, are agateagate
, translucent, cryptocrystalline variety of quartz and a subvariety of chalcedony. Agates are identical in chemical structure to jasper, flint, chert, petrified wood, and tiger's-eye, and are often found in association with opal.
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, bloodstone, carneliancarnelian
or cornelian
, variety of red chalcedony, used as a gem. It is distinguished from sard by the shade of red, carnelian being bright red and sard brownish. The red coloring is apparently caused by iron oxide.
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, chrysoprase, jasperjasper,
opaque, impure cryptocrystalline quartz, usually red, but also yellow, green, and grayish blue. It is used as a gem. Ribbon jasper has the colors in stripes.
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, onyxonyx
, variety of cryptocrystalline quartz, differing from agate only in that the bands of which it is composed are parallel and regular. Its appearance is most striking when the bands are of sharply contrasting colors; black and white specimens are often used for cameos.
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, sard, and sardonyx.



a mineral, a cryptocrystalline variety of quartz. Chalcedony contains various admixtures, including Fe3+, Al3+, and up to 1–1.5 percent water. When viewed under the microscope, it reveals a slender fibrous, frequently divergent, structure. The fibers of quartz microcrystals are sometimes spiraled, as in rainbow chalcedony, which exhibits an iridescence. Chalcedony forms incrustations, reniform, botryoidal, stalactitic, or sinter aggregates, veins, concretions, and pseudomorphs after organic remains and minerals. It also fills cavities.

Chalcedony is translucent to transparent and has a waxy luster when fractured. Occurring in many different colors, it has no cleavage and an uneven or hackly fracture. It has a hardness of 6.5–7 on Mohs’ scale and a density of 2,570–2,640 kg/m3.

Numerous varieties of chalcedony are identified according to various characteristics. Varieties recognized on the basis of color differences include chrysoprase (green), sard (red-brown), carne-lian (reddish yellow), sapphirine (bluish gray), plasma (an onion-green cloudy chalcedony), and cer-agate (light yellow). Varieties with a spotted coloration include heliotrope (green with red spots), myrickite (gray with red spots), and point agate, or point chalcedony (white or light gray flecked with red points).

Varieties of chalcedony distinguished according to structure include various agates, which have concentric bands, and onyx, or banded agate, which has parallel straight or curved bands. Uruguayan agates combine concentric and horizontal bands. Various types of agates are distinguished according to the design of the bands, for example, fortification agate, ruin agate, landscape agate, star agate, and cloud agate.

Varieties distinguished according to inclusions include moss agate, a translucent variety with inclusions of green chlorite or actinolite, mocha stone (dendritic agate), which has dendritic inclusions of manganese or iron oxide, and enhydros, a variety consisting of nodules and containing water-filled cavities.

Chalcedony is a widely occurring rock-forming mineral. It is formed from low-temperature hydrothermal solutions, as well as during the processes of diagenesis, epigénesis, and weathering. It precipitates out primarily from colloidal solutions, products of the recrystallization of silica gels. Chalcedony is often found in sedimentary rocks in the form of nodules, concretions, sheet deposits, and pseudomorphs after shells, corals, and the like. Chalcedony is the primary component of many silica rocks, petrified wood, and jasper. The chief industrial deposits of chalcedony are associated with amygdaloidal effusive rocks and products of their disintegration.

Because of their diversity of color and ability to take a good polish, chalcedony and its varieties have been used since ancient times as gemstones and for making ritual objects and ornaments. Neolithic chalcedony and carnelian beads have been found, as well as chalcedony and carnelian cylindrical seals (Babylonia) and agate and carnelian insets in ancient Egyptian gold ornaments. Onyx was widely used in the ancient states of Middle Asia and the Mediterranean region.

Agate and chalcedony are also used in laboratory work for chemical and pharmaceutical purposes (agate mortars and pestles) and in precision instrument-making (reference prisms of weights and the like). In the USSR, deposits of gem-quality chalcedony and its colored varieties are found in Eastern Transbaikalia (Tuldun), the Urals (Magnitogorsk), Transcaucasia (Akhaltsikhe), and the Crimea (Karadag).


Mineraly: Spravochnik, vol. 2, fasc. 2. Moscow, 1965.
Kievlenko, E. Ia., and N. N. Senkevich. Geologiia mestorozhdenii podelochnykh kamnei. Moscow, 1976.
Dana, J. D., E. S. Dana, and C. Frondel. Mineraly kremnezema. Moscow, 1966. (Sistema mineralogii, vol. 3.) (Translated from English.)



A cryptocrystalline variety of quartz; occurs as crusts with a rounded, mammillary, or botryoidal surface and as a major constituent of nodular and bedded cherts; varieties include carnelian and bloodstone.


A submicroscopic variety of fibrous quartz, generally translucent and containing variable amounts of opal; reacts with alkalies in portland cement.


a microcrystalline often greyish form of quartz with crystals arranged in parallel fibres: a gemstone. Formula: SiO2
References in periodicals archive ?
It has a relatively non-uniform microstructure, comprising a fine-grained groundmass (70%) of quartz crystals (up to 5 [micro]m across) with scattered abundant small chalcedonic areas (25%), calcite crystals (4%) and tiny opaque minerals (1%) (FIGURES 5A & 5B).
These silcretes are attributed to brackish, silica-saturated groundwaters from weathering granites, with precipitation of opaline and chalcedonic silica occurring in response to evaporative concentration as the flow path approaches a major discharge zone.
Although each site makes use of local raw materials, the people were also obtaining stone from certain specific, more distant sources, namely Rum bloodstone and baked mudstone and chalcedonic silica from Staffin.
Furthermore, KA has observed that quarries of white chalcedonic silcrete on Nicholson station were important sources of material that was heated to make exceptionally large (>8 cm) Kimberley points (Figure 5).
The vein gangue is comprised of equal proportions of chalcedonic quartz and feldspar, and the main sulphide constituent is pyrite (5% to 15%) with lesser galena, sphalerite and chalcopyrite.
Strong evidence of epithermal/mesothermal mineralisation and alteration in the Kwaiahia region was noted with float rocks showing features such as fine-grained chalcedonic quartz + grey fine sulphides (mainly pyrite) with silvery grey to black minerals (stibnite), weakly banded chalcedonic quartz/ calcite and vuggy and lattice textures (bladed calcite replaced by silica).
23) Rather than using the Chalcedonic formula of Jesus Christ's full divinity and full humanity as a starting point, theologians of this post-Vatican II generation turn to the scriptural narratives about Jesus' historical ministry.
This zone consists of both a calc-silicate skarn system as well as chalcedonic and quartz veining with arsenopyrite and pyrhhotite in coarse clastic sediments.
03 opt; whereas drill hole MS-2 hit a 40-foot thick zone of chalcedonic veining at 160-200 feet, which ran 0.
The Knoll Zone The Knoll Zone is characterized by outcropping silicified epiclastic sediments with chalcedonic coliform quartz veining, which likely represent the same sequence of epiclastic sediments encountered in the weakly mineralized hangingwall at the Carlos Zone.
These intercepts are related to chalcedonic veins and veinlets hosted within andesitic rocks.
Veins associated with higher gold values, (greater than 1 g/t Au), generally consist of fine grained or chalcedonic quartz with weak to moderate banding.