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Chalcedony | Variety of Quartz | Agate | Onyx | Jasper | Fire Agate
History and General Information of Chalcedony Quartz Gemstones

A variety of mineral quartz that is found in myriad colors is collectively referred to as Chalcedony. Often Chalcedony is known by its popular names rather than this name, which refers to the brand group of microcrystalline quartz. It can be found in any shape, size and form. Due to its extreme hardness it is used for a variety of purposes, ranging from jewel making to being used as a decorative stone, used as embellishment for homes. 
In the gem trade, the term Chalcedony is used to refer to the white or lightly colored pieces.  Specimens with colored bands are called agates, while chalcedony in conjunction with other minerals is named differently, like onyx, carnelian, mocha stone and bloodstone. It is found in volcanic and sedimentary rocks in the form of rounded crusts, stalactites and rinds.
In ancient times, especially among the Romans, it was preferred in making official seals while the Victorians used it to make cameos and intaglios. This stone finds great use in fashioning compresso or the Florentine mosaic. This is a decorative technique, wherein pictures are created through the use of thin, shaped pieces of stones of all colors. This work was widely popular in 16th century Florence.

Chemical Composition of Chalcedony

It is chemically represented as SiO2, being a variety of quartz.
Varieties of Chalcedony
The main varieties of chalcedony are

•    Agate - Has curved or angular bands or layers

•    Milky Chalcedony - Semitransparent to translucent, nearly colorless or white

•    Carnelian - Semitransparent to translucent , yellow-orange to orangy red, brownish red to brownish orange

•    Onyx which has straight, parallel layers of different colors

•    Carnelian onyx - Onyx showing bands of carnelian colors alternating with either black or white parallel bands

•    Jasper – General variety term applied to semitransparent to opaque chalcedony of any color or combination of colors, except solid black or specially labeled material such as bloodstone, onyx etc

•    Plasma – Semitranslucent to opaque dark green with white or yellowish spots

•    Amethystine chalcedony – Semitranslucent to opaque purple, also marketed under the name damsonite

•    Chrysoprase - Semitransparent to translucent, light to medium yellowish green (apple green color)

•    Bloodstone ( Heliotrope) – Semitranslucent to opaque, dark green with red to brownish red spots

•    Prase – Translucent to semitranslucent, with a generally darker, less saturated green than in chrysoprase

•    Landscape agate, picture agate, fortification agate etc – Chalcedony with colored patterns resembling a landscape etc  

•    Moss Agate or Mocha Stone – Chalcedony with green, black or brown inclusions that look like moss

•    Dendritic Agate – Chalcedony with dark inclusions resembling tree branches

•    Fire Agate - Semitranslucent to opaque, with iridescent colors against a brown bodycolor

•    Iris Agate - Semitransparent to translucent, with iridescent colors ( phenomenon best seen in transmitted light on thin slices)

•    Petrified wood, agatized wood, silicified wood – wood that has been entirely replaced by chalcedony, it resembles patterned jasper

•    Sard - Semitransparent to translucent, dark brownish red to brown to dark orange (darker and less saturated colors than those in carnelian)

•    Sardonyx – Onyx showing bands of sard colors alternating with either black or white parallel bands

•    Chrysocolla Chalcedony or Chrysocolla Quartz – Translucent to semitranslucent, intense light blue or blue green, colored by the copper mineral chrysocolla

Physical Properties of Chalcedony

It has a hardness of 6.5 on the Moh’s scale and a specific gravity of 2.58 to 2.64 with a refractive index of 1.544 to 1.533. It has a hexagonal structure with conchoidal fracture and no cleavage. It can be transparent and opaque with a dull, waxy, vitreous luster.  It is also piezoelectric.

Color of Chalcedony

Chalcedony is found in almost all colors and due to its porous nature, it can be dyed into any preferred color. Most colored semiprecious stones are some variety of Chalcedony and because of this a collector can find any colour he wishes to acquire.

Cut

It is generally cut and polished into round shapes to be used in making different kinds of jewelry.  It is more often used for carving and decorative purposes.

Carat

Being a commonly found stone, and occurring in a number of varieties, Chalcedony is found in the smallest to the biggest size and very often it is sold as slabs and by the ton to collectors.

Clarity

This group of gemstones are semi transparent to translucent to opaque . This stone often has impurities which may form a moss like growth which is often termed as the moss agate.  In Chalcedony with manganese oxide impurities, called dendritic agate, tree formation like images can be seen. Please refer below for detailed classifications.

Price of Chalcedony

Due to the wide variety of the stone available in the market, it is priced rather inexpensively, but stones with striking features, good color and clarity command high prices.

Producers of Chalcedony

It occurs in many countries round the world, but the finest specimens come from India, Burma, Brazil, Mexico, USA, Madagascar, Austria, Iceland, New Zealand, Russia and Britain.

Simulants

Due to the wide variety of stones which come under this nomenclature, the stone has many simulants.

Interesting Trivia and other Chalcedony Facts


Chalcedony is used in healing people who suffer from hysteria, sadness, depression or any kind of mental illness as it brings about a sense of calm. It is also helpful in preserving good eyesight and helps to reduce fever. It is used as a cleanser of wounds, especially open sores. It absorbs and dispels all kinds of negativity, and makes a person amenable and generous.  



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