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GEOLOGY ARTICLE

Agates
by WC McDANIEL

06.21.00: Agates are a microcrystalline variety of the quartz group of varying colors, a chalcedony. Agates are usually translucent in thin slices, otherwise opaque. Agates occur as spherical or almond shaped inclusions in basic volcanic rock. The stripey appearance is the result of rhythmic crystallization. Under the influence of weathering a white crust forms on the outer layer and in the uppermost agate bands. In the interior of the agate-almonds there are frequently well-developed crystals such as rock crystal, amethyst, smoky quartz, calcite, hematite, or siderite. Agates can be found in Brazil, Uraguay, China, India, Madagascar, Mexico, and the United States.

Many agates are normally an unattractive grey color and are only feebly patterned. They assume their brightly colored appearance and striking structures only by being dyed. The extent to which the individual layera take on color varies according to porosity, water content, and crystallinity.

There are many varied uses of agates: objets d'art, ring-stones, brooches, pendants, as the base-stone for intaglios and cameos, and also a widely varied use in technology because of its toughness and chemical resistance. There are numerous variety names depending on color and pattern.

Banded agate: Layers parallel to the outer surface
Enhydros (waterstone): Agate-almond full of water which shines through the walls. Quickly dries out in air.
Fortification agate: Patterned like the bastions of an old fortress.
Tubular agate: Agate shot through with tube-like feeder canals.
Sard stone: Agate with straight interior layers.
Brecciated agate: Broken agate later naturally cemented.

In its wider sense chalcedony includes microcrystalline quartz (agate, true chalcedony, chrysoprase, dendritic agate, heliotrope, hornstone, jasper, carnelian, moss agate, onyx, sard); in a narrower sense only the grey-blue variety.

Composition: SiO2
Hardness: 6-1/2 to 7
Specific gravity: 2.58-2.64
Cleavage: none
Fracture: uneven, conchoidal
Color: bluish, white-grey
Streak: white
Lustre: waxy, dull
Transparency: translucent
Crystal system: trigonal

This information was gathered from the Handbook of Rocks Minerals & Gemstones by Walter Schuman, 1993