Agate

Agate

Agate (ăg'ĭt) , 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. The colorful, banded rocks are used as a semiprecious gemstone and in the manufacture of grinding equipment. An agate's banding forms as silica from solution is slowly deposited into cavities and veins in older rock. The stones can be artificially stained to produce combinations of color more vivid than those found in the natural state. The cutting and staining of agates has long been centered at Idar-Oberstein, Germany. Important sources of agate are Brazil, Uruguay, and the United States (Oregon, Washington, and around Lake Superior). The moss agate or mocha stone contains visible impurities in the form of dendritic shapes that resemble moss. See onyx.

Types of agate

A Mexican agate, showing only a single eye, has received the name of "cyclops agate." Included matter of a green, golden, red, black or other colour or combinations embedded in the chalcedony and disposed in filaments and other forms suggestive of vegetable growth, gives rise to dendritic or moss agate (named varieties include Maury Mountain, Richardson Ranch, Sheep Creek and others). Dendritic agates have beautiful fern like patterns on them formed due to the presence of manganese and iron ions. Other types of included matter deposited during agate-building include sagenitic growths (radial mineral crystals) and chunks of entrapped detritus (such as sand, ash, or mud). Occasionally agate fills a void left by decomposed vegetative material such as a tree limb or root and is called limb cast agate due to its appearance.

Turritella agate is formed from fossil Turritella shells silicified in a chalcedony base. Turritella are spiral marine gastropods having elongated, spiral shells composed of many whorls. Similarly, coral, petrified wood and other organic remains or porous rocks can also become agatized. Agatized coral is often referred to as Petoskey agate or stone.

Certain stones, when examined in thin sections by transmitted light, show a diffraction spectrum due to the extreme delicacy of the successive bands, whence they are termed rainbow agates. Often agate coexists with layers or masses of opal, jasper or crystalline quartz due to ambient variations during the formation process.

Other forms of agate include carnelian agate (usually exhibiting reddish hues), Botswana agate, Ellensburg blue agate, blue lace agate, plume agate (such as Carey, Graveyard Point, Sage, St. Johns, Teeter Ranch and others), tube agate (with visible flow channels), fortification agate (which exhibit little or no layered structure), fire agate (which seems to glow internally like an opal) and Mexican crazy-lace agate (which exhibits an often brightly colored, complex banded pattern).

Agate beliefs

In many traditions agate is believed to cure the stings of scorpions and the bites of snakes, soothe the mind, prevent contagion, still thunder and lightning, promote eloquence, secure the favour of the powerful, and bring victory over enemies. Persian magi are also known to have prized agate rings in their work and beliefs.


Industry uses agates chiefly to make ornaments such as pins, brooches, paper knives, inkstands, and seals. Because of its hardness and ability to resist acids, agate is used to make mortars and pestles to crush and mix chemicals.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article Agate.