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Chemical Formula | SiO2 | |
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Composition | Silicon dioxide | |
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Color | Occurs in every imaginable color. The variety Agate is banded in many different color combinations. | |
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Streak | White | |
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Hardness | 7 | |
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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. | |
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Transparency | Transparent to opaque | |
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Specific Gravity | 2.6 - 2.7 | |
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Luster | Vitreous, waxy, or dull. Some yellow or brown varieties are resinous. | |
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Cleavage | None | |
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Fracture | Conchoidal | |
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Tenacity | Brittle | |
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Other ID Marks | 1) Many specimens fluoresce, usually white or green 2) Triboluminescent 3) Piezoelectric |
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Other Names |  Chalcedonyx, Chalcedon,
Chalcedonite Microcrystalline
Quartz or Cryptocrystalline Quartz
(distinguishes Chalcedony from crystalline Quartz) |
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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. (Quartz and Chalcedony variety page) 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. |
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Polymorphs | See Polymorphs of SiO2 See also: Quartz Opal Melanophlogite |
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In Group | Silicates ; Tectosilicates ; Silica group May also be classified as an oxide (Oxides ; Simple Oxides) |
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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 only white or
lightly 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. |
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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. | |
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Striking Features | Hardness and form | |
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Complex Tests | Dissolves in hydrofluoric acid | |
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Popularity (1-4) | 1 | |
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Prevalence (1-3) | 1 | |
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Demand (1-3) | 1 | |
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Distinguishing Similar Minerals |
The hardness and crystal habits of Chalcedony distinguish it from all minerals. | |
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Commonly Occurs With |
Chalcedony occurs in virtually all mineral environments, and it may be associated with almost every mineral. | |
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Noteworthy Localities |
Worldwide. There are literally thousands of
localities where fine specimens have come. Only a select
few are mentioned here. The first worked Agate deposits were near Idar-Oberstien, Germany, where Agate Geodes of unique habit were found. That deposit has long since been worked out. The most abundant source of Agate Geodes is the Rio Grande do Sul in Brazil, where many different sizes and types occur. These Agates are easily dyed. An interesting Agate Geode type, known as a "Thunder Egg" can be found in large amounts in Jefferson Co., Oregon. Commercial Agate deposits also exist in China, India, Madagascar, Mexico, and on the shores of Lake Superior. "Fire Agate" comes from Mexico and Deer Hill, Arizona. |
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