About
Most mineral reference guides list Chalcedony as an individual mineral, but in reality it is a variety of Quartz. It is the microcrystalline form of Quartz, forming only occurs in microscopic, compacted crystals. This page deals only with the crystalline forms of Quartz. Chalcedony is listed on its own dedicated page in this guide. Other important varieties of Quartz, such as Amethyst, Citrine, and Agate, also have dedicated pages due to their popularity and individual varieties.
Some forms of Quartz, especially the gemstone forms, have their color enhanced. Almost all forms of the yellow-brown variety Citrine are in fact heat treated. Much Amethyst is also heat treated to intensify color, and a green transparent form known as "Green Amethyst" or "Prasiolite" is formed by heat treating certain types of Amethyst. There is also a transparent sky blue form of Quartz crystals, as well as a wildly iridescent type that are synthetically colored by irradiation of gold. In some localities, Hematite forms a thin red or brown layer internally in the Quartz crystal, giving it a natural bright red to brown coloring, and sometimes even a mild natural iridescence.
Quartz frequently forms the inner lining of geodes. Most geodes have an inner layer of larger crystalline Quartz, and an outer layer of Chalcedony or banded Agate.
For additional information, see the gemstone section on Quartz.
Crystal Forms & Aggregates
Crystals frequently twin; a famous twinning habit is the Japanese twin, where two crystals contact at a 90º angle. Quartz crystals may also contain a scepter growth, where the top of a crystal bulges out from the rest of the crystal, and may also form as phantom growth, where one crystal forms over another, leaving a ghosted form inside.
The crystal structure of Quartz is a very complicated. As a result of a changeover from alpha to beta Quartz, crystals form as hexagonal prisms with modified crystal faces.
Click here for a detailed explanation on the crystal structure of Quartz.
Other ID Marks
2) Triboluminescent.
3) Piezoelectric.
Complex Tests
Striking Features
Environment
Varieties
✓ common · ✗ uncommon
- {%Amethyst%}— Purple variety of Quartz. For additional information, see the mineral page on Amethyst.
- {%Chalcedony%}— Microcrystalline form of Quartz. For additional information, see the mineral page on Chalcedony.
- {%Citrine%}— Yellow, orange, or brown variety of Quartz. For additional information, see the mineral page on Citrine.
- Aventurine— Opaque form of compact Quartz or Chalcedony containing small Mica, Hematite, or Goethite scales which cause a glistening effect. Although technically Aventurine is classified as rock due to its composition of several minerals, it most often is regarded as a variety of Quartz or Chalcedony. For additional information, see the gemstone page on Aventurine.
- Milky Quartz— White, translucent to opaque variety of Quartz.
- Rock Crystal— Colorless, transparent variety of Quartz in large crystal form.
- Rose Quartz— Pink variety of Quartz. For additional information, see the gemstone page on Rose Quartz.
- Rutilated Quartz— Quartz with golden yellow, needle-like Rutile inclusions. For additional information, see the gemstone page on Rutilated Quartz.
- Smoky Quartz— Brown to black, "smoky" variety of Quartz. For additional information, see the gemstone page on Smoky Quartz.
- Aqua Aura— Quartz synthetically enhanced with a coating using gold (and sometimes other metals) to give it a neon blue or other neon color.
- Blue Quartz— The blue variety of Quartz, which is very uncommon in nature and rarely in crystal form. Most "Blue Quartz" is what is popularly known as "Aqua Aura", essentially clear Rock Crystal synthetically irradiated with gold to form a deep sky blue color. Blue Quartz may also refer to a dull grayish-blue Quartz in massive form with Crocidolite inclusions.
- Cactus Quartz— Form of Quartz, usually Amethyst, Citrine, or a combination of the two, that contains a large crystal or crystals overgrown with a layer of spiky smaller crystals. Cactus Quartz is specific to Boekenhouthoek (Magaliesberg) in South Africa.
- Faden Quartz— Group of Quartz crystals with a white thread-like zone running through the interior, with the crystals having formed around the thread axis.
- Ferruginous Quartz— Quartz with an opaque red to brown Hematite coating or internal inclusion.
- Herkimer Diamond— Exceptionally lustrous and clear Quartz crystals from the Herkimer Co. vicinity in the Mohawk Valley region of Central New York State. Herkimer Diamond crystals are usually doubly terminated and short.
- Morion— Opaque form of black Quartz. A type of Smoky Quartz.
- Phantom Quartz— Quartz containing internal phantom growths, or ghostlike layers within a crystal.
- Prase— Light to emerald green, transparent to translucent Quartz, with coloring caused from inclusions of green minerals, such as Actinolite, Hedenbergite, Chlorite, or Malachite.
- Praseme— Light green, translucent form of Quartz with Hedenbergite inclusions found on Serifos Island, Greece.
- Prasiolite— Prasiolite describes a light green Quartz artificially colored by heat treatment of certain types of Amethyst. May also be called Green Amethyst by some jewelers.
- Rainbow Quartz— Quartz synthetically colored with an iridescent layer formed from gold or other metals. Also see Aqua Aura.
- Scepter Quartz— Quartz crystal with a scepter like protrusion on the end of the crystal that is wider than the rest of the crystal.
- Star Quartz— Polished Quartz displaying asterism in the form of a six-rayed star.
- Tourmalinated Quartz— Quartz with splintery Tourmaline inclusions.
Polymorphs
Uses
In addition to all the practical uses, Quartz is essential to the gem trade. (See the separate Quartz gemstone page.) Many varieties are faceted as gems. Amethyst and Citrine are the most well-known gem varieties. Rose Quartz, Smoky Quartz, Rock Crystal, and Aventurine are also cut or polished into gems. Small colorless Quartz crystals are worn by some as pendants for good luck.
Quartz is also a very popular among collectors. Certain collectors specialize their collection entirely on Quartz alone.
Noteworthy Localities
Some of the largest and most impressive Quartz crystals, weighing many tons and yet still perfectly formed, come from several pegmatite districts in the state of Minas Gerais, Brazil. A well-known French locality of excellent Rock Crystal clusters is the La Gardette Mine, Bourg d'Oisans, Isere.
In the U.S., flawless Rock Crystal of exceptional quality and abundance comes from Arkansas, most notably in the Hot Springs area, Garland Co.; and Mount Ida in the Ouachita Mountains, Montgomery Co. Dense, nail-like clusters of clear Quartz crystals are classics at the Jeffrey Quarry, Pulaski Co.
Lustrous, doubly terminated, and often stubby crystals come from the Mohawk Valley region of central New York, in an area covered by parts of Herkimer, Montgomery, and Fulton Counties. Quartz crystals from this region are affectionately known as Herkimer Diamonds, and are highly collectible and very popular with both novice and expert collectors. Specific occurrences within this region include the Ace of Diamond and adjacent Herkimer Diamonds Mine Resort in Middleville, Herkimer Co.; Diamond Acres and Hickory Hill, near Fonda, Montgomery Co.; and Crystal Grove, near Lassellsville, Fulton Co. Two other important localities in the region that are no longer active producers are the Benchmark Quarry, St. Johnsville, Montgomery Co.; and the Treasure Mountain Mine, Fall Hill, Little Falls, Herkimer Co.
Although Rose Quartz is very common in massive habit, good crystals are rare and only found in a few localities. These are mostly in Minas Gerais, Brazil, in Galileia (especially at Lavra da Pitorra); and in Itinga, in the Jequitinhonha Valley; as well as at Newry, Oxford Co., Maine.
Fine Smoky Quartz comes from the Pikes Peak area, El Paso Co., Colorado. The Alps contain two very classic occurrences in Switzerland in St. Gotthard, Uri, Switzerland; and in Chamonix-Mont Blanc, France. The finest Rutilated Quartz comes from Novo Horizonte and Ibitiara, Bahia, Brazil. Tall prismatic Quartz colored dark green from Hedenbergite inclusions occur at Serifos Island, Greece. Exceptional Faden Quartz can be found at the Dara Ismael Khan District, Waziristan, Pakistan.
Common Mineral Associations
Distinguishing Similar Minerals
Feldspars - Softer (6), perfect cleavage.
Calcite - Much softer (3).
Photos
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