Chalcedony

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Mogánite-798422

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Quartz - Agateplate, redbrown-white

carnelian sard (mineral specimen)

Chryzopras Polsko

Agate-180670

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (silicon dioxide).

Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white to gray, grayish-blue, or a shade of brown ranging from pale to nearly black. The color of chalcedony sold commercially is often enhanced by dyeing or heating.

The name "chalcedony" comes from the Latin chalcedonius (alternatively spelled calchedonius). The name is probably derived from Chalcedon, an ancient Greek town in Asia Minor, in modern-day Turkey, although it is likely that the mineral referred to by this name in classical antiquity was actually a different mineral, such as alabaster, onyx, or sard.

Varieties

Chalcedony includes a wide variety of gemstones, including:

• Agate: Chalcedony with multi-colored layers or bands.
• Onyx: Agate where the bands are straight, parallel and consistent in size.
• Jasper: Opaque chalcedony, usually red, yellow, brown or green in color.
• Carnelian: Reddish-orange chalcedony.
• Chrysoprase: Green chalcedony, colored by nickel oxide.
• Heliotrope (Bloodstone): Dark green chalcedony with red spots.
• Moss agate: Chalcedony with moss-like inclusions.

Formation and Occurrence

Chalcedony forms in volcanic and sedimentary rocks by the deposition of silica from solution in cavities and fractures at low temperatures, often below 100°C. It is commonly found in association with other silica-rich rocks such as granite, gneiss, and basalt. Chalcedony is also a common material in sedimentary rocks, where it can form as nodules or layers.

Uses

Historically, chalcedony was used in jewelry and for carving into cameos, intaglios, and other decorative items. Its fine texture and durability make it suitable for intricate carving. Today, it is still used in jewelry and is also popular among collectors of gemstones and minerals.

Care and Maintenance

Chalcedony is relatively hard, rating 6 to 7 on the Mohs scale of mineral hardness, but it can still be scratched by harder materials. It should be stored separately from other gemstones to avoid scratches. It can be cleaned with mild soap and water, but harsh chemicals should be avoided.