Quartz (silicon dioxide/silica - SiO2) is the most common mineral in the Earth's crust.  It is composed of the two most abundant elements in the crust - oxygen and silicon.  It has a glassy, nonmetallic luster, is commonly clearish to whitish to grayish in color, has a white streak, is quite hard (H≡7), forms hexagonal crystals, has no cleavage, and has conchoidal fracture.  Quartz can be any color: clear, white, gray, black, brown, pink, red, purple, blue, green, orange, etc.



Quartz (2.2 cm across) - clear quartz crystal ("rock crystal").



Quartz - nice radiating cluster of clear quartz crystals from the Collier Creek Mine, Montgomery County, Arkansas, USA.



Quartz (above & below) - doubly terminated clear quartz (“Herkimer Diamond Quartz”) from dolostone vugs.

Stratigraphy: Little Falls Formation, Upper Cambrian.

Locality: near Herkimer, New York State, USA.

Above: 2.6 cm across.

Below: stacked “Herkimer Diamonds”.



Capped quartz from St. Dennis, Cornwall, England.

(Wayne State University collection, Detroit, Michigan, USA)



Quartz with phantom from Minas Gerais State, Brazil.

(Wayne State University collection, Detroit, Michigan, USA)



Quartz showing Japan-law twinning from the Otome Mine near Kohfu, Yamanashi Prefecture, Japan.  Twinning refers to the intergrowth of two or more crystals of the same mineral.  A Japan-law twin is one of several types of contact twins, characterized by having an obvious contact plane separating the crystals.  (HMNH # 28144, Harvard Mineralogical Museum, Harvard Museum of Natural History, Harvard University, Cambridge, Massachusetts, USA)



Quartz geode (above & below; above: 9.3 cm across at its widest; below: field of view ~3.8 cm across) - geodes form by precipitation of crystals in cavities by fluids moving through rocks.

Concentrically-layered microcrystalline quartz (“agate”) lines the walls of the geode, and is capped by clearish macrocrystalline quartz.




Quartz after fluorite from the Idarado Mine, Telluride Mining District, San Miguel County, Colorado, USA.  The cubic shapes are leftover from the original fluorite crystals - quartz has replaced them.

(CSM # 51991, Colorado School of Mines Geology Museum, Golden, Colorado, USA)



Most color varieties of quartz result from silica mixed with impurities.  Sometimes, coloration is provided by radiation scorching or by the presence of abundant fluid inclusions.



Milky quartz (4.3 cm across) - the white coloration is attributed to the presence of abundant microscopic fluid inclusions.



Milky quartz from the Ohio Mine near Ouray, Colorado, USA.  (Cranbrook Institute of Science specimen, Bloomfield Hills, Michigan, USA)



Rose Quartz (3.6 cm across) - the pink coloration of rose quartz has been attributed to the presence of Ti+4 impurity or the presence of dumortierite-like fibers.  Some rose quartz coloration has been attributed to radiation bombardment of quartz having Al and P impurities.



Amethyst (purple quartz) - the coloring agent for amethyst is not agreed upon.  Some say its due to Fe+4, some say Fe+3, some say Mn.  The cluster of amethyst crystals shown above is from Russia.   (CM public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA)


Amethyst (purple quartz) in large geode ("amethyst cathedral") from South America.


Amethyst (purple quartz) lining cavity from Rio Grande do Sul, Brazil (Colorado School of Mines Geology Museum, Golden, Colorado, USA).


Amethyst (purple quartz) from Vera Cruz, Mexico (Wayne State University specimen, Detroit, Michigan, USA).



Smoky quartz in geode - smoky quartz is blackish or very dark gray or brownish.  The coloration is attributed to radiation scorching or the presence of FeO or Mn impurities.  Smoky quartz can also be simulated by artificial irradiation of clear quartz.

Locality: Sowerbutt Quarry, Prospect Park, New Jersey, USA.



Chrysoprase (3.0 cm across along the base) from Queensland, Australia.  Chrysoprase is an apple-green, microcrystalline "variety" of quartz.  The green color has been attributed to nickel oxide impurity.



Jasper (4.2 cm across along the base) from Cave Creek, Arizona, USA.  Jasper is an intensely red, micro- to cryptocrystalline “variety” of quartz.  The red is from hematite (Fe2O3).  Jasper is really not a variety of quartz - it's a variety of chert, a sedimentary rock composed of cryptocrystalline quartz.



There are numerous named "varieties" of quartz.  Many of the names are inconsistently applied by the general public and by geologists.



Agate (cut & polished slice, 7.4 cm across) - the term agate is usually used to refer to irregularly- & concentrically-layered masses of microcrystalline quartz.  Individual layers consisting of microcrystalline, fibrous quartz (either translucent or opaque) are said to be chalcedony.  The different layers vary in color due to the presence of various impurities.  Many agate masses are simply geodes that have completely filled up with quartz.  Common agate colors are clearish-whitish-grayish, brownish-red, and yellowish-brown.  Commercial agates that occur in greens and blues and purples are almost always dyed (faked).



Agate (11.0 cm across) - cut & polished nodule.



Agate-filled geodes (above & below) from the Las Choyas Geode Deposit in northern Aldama County, north-central Chihuahua State, northern Mexico.  Geodes at this locality occur in structurally-folded, rhyolitic volcanic tuffs (ash flow tuffs) of Middle to Late Eocene age (~35-44 m.y.).  The geodes were originally cavities in the rhyolitic rock.  These cavities (lithophysae) formed before the rock completely lithified.  The original ash flow deposit had some subspherical structures known as spherulites, composed of glassy to cryptocrystalline material (many felsic extrusive igneous rocks have these).  Expanding gases in the spherulites destroyed the material, resulting in empty spaces.  In the near-latest Eocene (~35 m.y.), regional rhyolite dome intrusions resulted in hot groundwater percolating through the rocks, leaching out silica and precipitating quartz in the lithophysae/cavities.

Specimens owned by Jeff Smith.

(Info. from Smith, 2010, The Las Choyas Geode Deposit, Chihuahua, Mexico, Rocks & Minerals 85: 112-122 and Keller, 1977, Quartz geodes from near the Sierra Gallego area, Chihuahua, Mexico, Mineralogical Record 10: 207-212.)



Agate from Adrasman, Tajikistan.  Specimen owned by Jerry Schaber.



Agate from Ankara, Turkey.  Specimen owned by Jerry Schaber.



Agate from Dryhead, Montana, USA.  Specimen owned by Jerry Schaber.



Agate from Madagascar.  Specimen owned by Jerry Schaber.



Agate from Madagascar.  Specimen owned by Jerry Schaber.



Agate from Nowy Kosicol, Poland.  Specimen owned by Jerry Schaber.



Agate from Rio de Sol, Brazil.  Specimen owned by Jerry Schaber.



Agate from Wave Hill, Australia.  Specimen owned by Jerry Schaber.



Agate from Wave Hill, Australia.  Specimen owned by Jerry Schaber.



Agate from Wave Hill, Australia.  Specimen owned by Jerry Schaber.



Holley Blue Agate” (top specimen is 3.9 cm across) - pale purplish chalcedony from Linn County, western Oregon, USA.  This attractive material is famous, but is never accompanied with useful information, in terms of specific locality or geologic context.  I don't know what the host rock is, or the stratigraphy, or the age.  I suspect that this chalcedony fills vesicles in Cenozoic-aged lava.  The locality is not specified anywhere in the literature.  The best locality information I came up with is “Holley Mountain” (wherever that is) near the towns of Holley or Sweet Home or Lebanon, Linn County, western Oregon, USA.

Specimens donated by James Cheshire.



Tiger-eye quartz (above & below; above: unpolished; below: polished) - tiger-eye quartz consists of orangish-brown or reddish-colored, compact masses of fibrous appearance that display chatoyancy (when tilted in the light, a moving wavy silky sheen can be seen).  It forms as a result of quartz replacing asbestiform amphibole.



Star quartz from Germany.  Each "star" is a radiating spray of twinned quartz crystals. (Wayne State University collection, Detroit, Michigan, USA)



Sometimes, quartz coloration is due to the presence of other minerals included within the crystal structure of quartz.



Quartz cluster colored blue by the presence of papagoite (CaCuAlSi2O6(OH)3 - calcium copper hydroxy-aluminosilicate); from Messina, South Africa.  (CM public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA)



Quartz (cut & polished slice) with inclusions of tourmaline (black lines & sticks) and hematite (scattered red spots) from Minas Gerais, Brazil.  (CMC RM 1146, Cincinnati Museum Center's rock & mineral collection, Cincinnati, Ohio, USA)



Rutilated quartz - quartz crystal with inclusions of long, pale-colored needles of rutile (TiO2).  (CM public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA)



Quartz with inclusions of asbestos from Savois Department, France.

(Wayne State University collection, Detroit, Michigan, USA)



Quartz with hematite inclusions (red) on sphalerite (black) from a skarn deposit in the Second Sovietskiy Mine at Dalnegorsk, Maritime Province, eastern Russia.



Photo gallery of quartz & its varieties



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