Formation & Sources of Chalcedony


Cryptocrystalline quartz is deposited at relatively low temperatures by circulating ground waters or by magmatic waters that have dropped to temperatures approaching those expected at the earth's surface. The earth's crust is so rich in silica that ground waters commonly approach saturation in silica content. As a result, chalcedony or other forms of quartz are often deposited in cavities in the near-surface rocks. Such waters may dissolve away pre-existing materials such as wood, bone and shells and replace them particle by particle with chalcedony. Chalcedony and/or crystalline quartz may fill or line cavities of any kind in near-surface rocks to form concretions or geodes.

Sources and Recovery of chalcedony

Because chalcedony is distributed widely throughout the world, any attempt to list and describe in detail all sources would be impractical in a work of this nature. Listed below, however, are some of the more notable localities.

White to pale grayish-blue chalcedony:- Siberia, Ice-land, India and the beach and desert regions of California.

Chrysoprase:- Australia, Silesia, California, Oregon, and India.

Moss agate:- India, Wyoming, Montana, Washington, Oregon, Texas and most western states.

Chrysocolla chalcedony:- Arizona, New Mexico and the Lake Superior region.

Carnelian and sard:- Brazil, India and Uruguay.

Bloodstone:- India, Siberia, Australia, Scotland and Brazil.

Prase:- Germany.

Plasma:- India, China and Germany.

 Banded agate and onyx:- Brazil, Madagascar and Uruguay.

Jasper:- Germany, Scotland, Sicily, Siberia, California, Oregon, Idaho, Texas, Montana and most western states.

Agatized woods:- Arizona, New. Mexico, California, Washington, Oregon and most western states.

Although chalcedony has the durability to make it common in alluvial deposits, profitable mining is usually confined to primary deposits. The low price of most varieties precludes the practicability of sorting gem gravels to select satisfactory pebbles. Since the bulk of the material is heated or dyed to improve color, masses of cheap gray agate are blasted out of extensive deposits for this purpose.

 

Free Web Hosting