About Chalcedony
Chalcedony is a microcrystalline form of quartz. Chalcedony comes in many different colors, textures and varieties. Although jewelers commonly refer only to its white form as chalcedony, it can be found in a variety of colors, which are known by other names such as onyx, agate and flint. An abundant mineral in many of its forms, it can found in many locations and has been used throughout recorded history for both decoration and utility.
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Chemical Composition and Properties
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Chalcedony, technically known as microcrystalline SiO2, silica dioxide, is a relatively hard mineral, with a hardness between 6.5 to 7. It has a transparent, vitreous, or resinous luster and a white streak. Its density is 2.61 gram per cubic centimeter, and it has a conchoidal (curved) fracture. It also has some special properties. Chalcedony will dissolve in hydrofluoric acid, and it is also piezoelectric, which means that it has a charge under stress.
History
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Chalcedony derives its name derived from Chalcedon, a port on the Sea of Marmara, where ornamental materials were mined and traded. Chalcedony seals from Minoan Crete date back to the Bronze Age. Early humans used chalcedony as knife tips, tools, bowls and cups. Due to its durability and ability to be polished, chalcedony has also been used ornamentally for hundreds of years.
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Varieties
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Although jewelers only refer to the white or light-colored variety as chalcedony, chalcedony can be found in many different colors including white, red, green and banded brown. Different colored varieties include flint, carnelian, agate, chrysoprase, onyx, and bloodstone.
Flint is a hard, easily fractured form of chalcedony that can be used to make sparks, thus its history as a fire starter.
Carnelian is translucent red. It is commonly used in rings and to make beads.
Agate is a banded form of chalcedony and comes in a number of unique varieties. For example, moss agate a green tree-like structure in the stone. Agates form from igneous rocks and are used primarily in jewelry.
Chrysoprase is an apple-green form of chalcedony that gets it's green color from nickel. It is highly prized and is of high value.
Onyx is white and black banded. It is formed from igneous rocks.
Bloodstone, commonly used in the Middle Ages, is dark green with flecks of red jasper throughout the stone.
Location
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Although the availability of the different forms of chalcedony varies, chalcedony can be found worldwide. Uruguay and Brazil have large deposits and Germany is is home to significant amounts of agate. Chrysoprase can be found in Germany, Russia and Eastern European countries, the United States, Canada and Brazil. Chalcedony can also be found in large supply in Turkey.
Uses
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Chalcedony is primarily used for jewelry and ornaments today. Both agate and onyx are used in carving cameos as the banding in color adds to the relief. Many other forms of chalcedony are carved into beads, made into cabochons or engraved. Historically, chalcedony was commonly used for bowls, cups, stone inlay and the base of goblets. It has also been found in ancient weapons.
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