CHALCEDONY - Microcrystalline variety of Quartz
Chalcedony - A Gem for the Ages

Chemical Formula	SiO2
Composition	Silicon dioxide
Color	Occurs in every imaginable color. The variety Agate is banded in many different color combinations.
Streak	White
Hardness	7
Crystal Forms and Aggregates	(Hexagonal) Chalcedony, being a microcrystalline variety of Quartz, does not occur in crystals. It occurs botryoidal, mammilary, stalactitic, massive, nodular, as smooth rounded pebbles, as banded masses, and in geodes.
Transparency	Transparent to opaque
Specific Gravity	2.6 - 2.7
Luster	Vitreous, waxy, or dull. Some yellow or brown varieties are resinous.
Cleavage	None
Fracture	Conchoidal
Tenacity	Brittle
Other ID Marks	1) Many specimens fluoresce, usually white or green 2) Triboluminescent 3) Piezoelectric
Other Names	Chalcedonyx, Chalcedon, Chalcedonite Microcrystalline Quartz or Cryptocrystalline Quartz (distinguishes Chalcedony from crystalline  Quartz)
Varieties	There are literally hundreds of variety names given to Quartz and Chalcedony. Some are not scientifically recognized, but are universally used. The important most important varieties are listed below. All other variety names, including seldom-used names, are listed in an individual page of Quartz and Chalcedony varieties. These are the main varieties: Agate - Banded variety Carnelian - Reddish, transparent to translucent variety Onyx - Banded variety in which the banding lines are straight and parallel, and consistent in band size. Jasper - Opaque variety of Chalcedony that occurs in all colors. Tiger's Eye - Pseudomorph of Quartz after fibrous Crocidolite. Chrysoprase - Apple-green variety Bloodstone (or Heliotrope) - Dark green to greenish blue variety dotted with small, red, bloodlike spots. Sard - Brownish to brownish-red, transparent to translucent variety Sardonyx - Banded variety with straight parallel bands of brownish to red alternating with white or black bands. Flint - Massive, uniformly colored variety that is somewhat impure. Scientifically, the term "Chalcedony" refers to any type of microcrystalline Quartz. However, in the gem trade it describes a common white or lightly colored nodular or massive type.
Polymorphs	See Polymorphs of SiO2 See also: Quartz Opal Melanophlogite
In Group	Silicates ; Tectosilicates ; Silica group May also be classified as an oxide (Oxides ; Simple Oxides)
All About	Chalcedony is not really known by its name. Scientist call it "Microcrystalline Quartz", and collectors and gem dealers call each variety by its particular name. There are hundreds of different Chalcedony varieties, and many have gem uses. In the gem trade, the name Chalcedony describes white, lightly ,blue ,dep blue colored nodular or massive Chalcedony. The variety Agate is most varied and popular of all Chalcedony. It is extremely abundant and creates unusual and stunning examples. Most Agate is ugly in a natural state. Specimens must be trimmed and polished to bring out their full beauty. Most specimens sold to collectors have already been treated, in the form of tumbled stones or polished slabs. Some Chalcedony is porous, and it can be dyed different colors. Many Agate specimens from Brazil are dyed, and are sold to the buyer without his knowledge of this fact. With experience, one can usually tell by the color of an Agate if it was dyed. Chalcedony sometimes occurs in geodes, lining the cavity with mammilary blobs. Agate sometimes completely fills a geode cavity, forming an Agate nodule (sometimes improperly called an "Agate Geode"). Most Agate on the market comes from these nodules. (More information on geodes) Chalcedony frequently pseudomorphs after organic materials. A well-known example is petrified wood, wood that has been completely transformed into Chalcedony (or one of several other minerals). In the Petrified Forest National Monument in Arizona, an entire forest was transformed into petrified wood. Remains of this ancient forest can be seen in the huge silicafied logs that are found in the area. Another well-known pseudomorph is Chalcedony after coral. Underneath the Tampa Bay in Florida, much coral has been replaced by Chalcedony, and its original form is preserved. Another Chalcedony pseudomorph is the famous "Tiger's Eye". This almost legendary mineral has a most interesting and confusing appearance. It is caused by a finely fibrous structure of Crocidolite that was once present, but has been transformed into Chalcedony through pseudomorphism. Impurities are frequently present in Chalcedony. They may form a moss like growth in the mineral, forming what is known as Moss Agate. Another example is Dendritic Agate, a variety of Chalcedony containing manganese oxide impurities that form fabrications resembling trees. The "trees" may be strikingly realistic.
Uses	Chalcedony is an important ornamental stone. The varieties Agate, Chrysoprase, Carnelian, Sard, Tiger's Eye, Bloodstone, Jasper, and Moss Agate are all carved into cabochons and beads, making fine gems. The apple-green variety, Chrysoprase, has a distinct color and commends a higher price than the other varieties. Agate is carved into cameos and ornamental objects, and is sometimes sold as polished slabs. Different varieties of Chalcedony are tumbled and polished, and sold to amateur collectors by the tons.
Striking Features	Hardness and form
Complex Tests	Dissolves in hydrofluoric acid
Popularity (1-4)	1
Prevalence (1-3)	1
Demand (1-3)	1
Distinguishing Similar Minerals	The hardness and crystal habits of Chalcedony distinguish it from all minerals.
Commonly Occurs With	Chalcedony occurs in virtually all mineral environments, and it may be associated with almost every mineral.

Technical information provided from http://www.minerals.net

Even in natural drusy, international competition is driving prices down. The author has observed that the market for unenhanced blue drusy is firm at about $0.90 per carat for custom-cut sizes and shapes. In past years, blue drusy material of this quality might have sold for as much as $3 to $5 per carat. The only segment of the market where prices for drusies, agates, and jaspers is still strong is the high end.

In chalcedony, on the other hand, the market is still growing. That popularity is reflected in price points, Randall says. "Prices have really taken off. Chalcedony rough from South Africa that I purchased 14 to 15 years ago for $500 per kilogram is $2,000 or more a kilo today, but the prices for cut stones have more than kept pace."
A common complaint in the industry is that while the cost of rough and prices for finished cabochons have been steadily increasing, the yield from rough has only increased slightly.

"Recoveries from chalcedony and scenic jaspers can be as low as 7 to 8 percent," says Penn. In faceted gemstones like amethyst or garnet, a custom cutter can often get a recovery of 15 to 20 percent, "Only small portions of a given piece of [chalcedony] rough may be usable for cutting quality cabochons."

Good blue and purple chalcedony is being mined in Turkey and several locations in Africa, including the hard to get Namibian material. Indonesia is producing purple chalcedony along with other colors, including yellow to deep orange. And the United States has active blue and purple chalcedony mines of its own, although the western part of the country is perhaps better known for its beautiful agates and jaspers.

West Coast Gemstones Inc.'s Dale Huett, who mines purple chalcedony in Nevada, says that he has been able to improve his yield through new mining techniques. "In the past 10 years, we have totally changed our mining methods. Using excavators, we are extracting larger, more solid pieces, and this style of mining is far more environmentally friendly. The better the quality of the rough that we mine, the better the price we can get, Amethyst Sage Agate at $1,000 per kilogram has to be quality rough, and our material is the best."

Other U.S. miners and most of the foreign ones have been slower in changing to larger-scale mechanized mining. Many are still digging with jackhammers and pry bars, while others drill and blast. These methods often result in more breaking and fracturing and the recovery of smaller pieces of rough. The use of hydraulic excavators for mining does require much higher up-front mining - costs, but long-term production and reclamation costs often are less, and the amount and quality of material recovered is much greater.

In the United States, the mining of the various types of chalcedony is transitioning from a hobbyist activity to a more professional effort. There have always been a number of professional miners involved in the recovery of chalcedony, but today their operations are becoming more mechanized, with more attention paid to the quality and size of the rough recovered.

Despite the demand in the high end of the market, and the increase in the price of chalcedony over the past few years, its popularity appears to be on a downswing. Just remember, though: Even when chalcedony in all its forms is down, it's never out.

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