AGATE (from the Greek word akhatēs * EN: agate; DE: Achat; FR: agate; ES: agata; RU: агат) is the mineral, the variety of the chalcedony with the banded or spotty texture, or with the decorative inclusions.
The evenly coloured agates (with the unclear banding) are: the carnelian is orange, pink, pinkish yellow, the sarder (sard) is brown, reddish brown, the carneol is meaty red, the sapphirine is grayish blue, etc. The banded agate is characterized by the alternation of the thin, smoothly curved concentric layers. The peculiar banded agate, the pictire of which is created by the sharp angular lines and is reminiscent of the contours of the bastions, the towers of the castles, the ruins of fortresses, is called the bastion (tower, fortification, fortress, ruins) agate. The layered agate is onyx, which is composed by the parallel planes of the alternating stripes (the carnelian onyx by the reddish brown and white, the carneol onyx by the red and white, the arabian onyx by the black and white); the rare rainbow (iridescent) agate is the finely banded (up to 600 stripes per 1 millimetre) one of its species. The iridescent plays of the colours are observed in the plates of this agate, which are less than 5 millimetres thick, and cut across the layers: the thin layers, acting like the diffraction grating, decompose the white light into the colours of the spectrum.
They distinguish among the spotted agates the heliotrope (the bloody jasper) with the characteristic red stripes and spots (mainly hematite or iron hydroxides) on the dark green opaque background, the mirikkit with cinnabar red spots on the gray background. The moss agate is the milky white or gray translucent chalcedony with the inclusions of the dendrites of the manganese and iron hydroxides, feather-like, fan-like, tubular, and other inclusions of the chlorite, actinolite, zeolites, and other minerals, which are creating the impression of the fern leaves, algae, moss, etc, which are included into the stone. Especially prized are the landscape agates with the tree-like inclusions, which are simulating the winter landscape. Sagenite agate contains the arrow-like inclusions of the rutile. The fire agate is the reddish brown chalcedony with the inclusions of the tiny crystals of the goethite, which are causing the red sparkling reflections. Enhydros are the small agate geodes with the cavity, which is filled by the remains of the mineral forming solution with the bubble of gas. The internal cavity in the agate geodes may be lined with the small crystals of the rock crystal, smoky quartz, or amethyst.
The hardness of the agate is about 7. The density is 2570-2580 kilograms per cubic metre. There are characteristic for the agate the conchoidal fracture, the waxy gloss, the high toughness, and the ability to get the perfect polish.
Agate is the hydrothermal post-volcanic mineral, which is filling the amygdale cavities and cracks in the volcanic rocks of the andesite-basalt and rarely of the rhyolitic composition. The eluvial (residual) agates in the ancient weathering crusts of the effusive volcanic rocks and the agates of the alluvial deposits have the great practical importance. The main deposits of agate are in Brazil, Uruguay, Argentina, and India. The famous deposits in Europe are in the FRG in the region of Idar-Oberstein; in the USSR, they are in the Lesser Caucasus (Akhaltsikhe group), in the East Siberia (Norsk, etc.) and at the North Timan.
Agate was known for a long time in the ancient Sumeria and Egypt; it was used for the manufacturing of the carved seals, amulets, and jewelry. The agate was especially widely used in the countries of the ancient Mediterranean region for the manufacturing of the gemstones (mostly the two colour onyx). The agate was also used as the material of the gemstones in the 16th-18th centuries. The agate, which has been polished as the cabochon, is included into the various jewelry and decorative items. They "ennoble" the non-impressive agates of the light gray colour, by colouring them artificially with the carbon or the salts of the various metals. The homogeneous agate with the unclear banding, because of its great strength, toughness, resistance to the abrasion, resistance to the aggressive environments, is used in the instrument manufacturing industry, precision engineering (the pivot bearings, the bearing prisms), in the textile industry (yarn feeders), for the manufacturing of the artificial fibers (spinnerets), for the manufacturing of the mortars, etc.
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