Quartz and Silica Minerals Gallery

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Different Kinds of Quartz

A quartz sampler
Quartz and Silica Minerals Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Quartz (crystalline silica or SiO2) is the most common single mineral of the continental crust. It is unusually hard for a white/clear mineral, hardness 7 on the Mohs scale. Quartz has a glassy appearance (vitreous luster). It never breaks in splinters but fractures in chips with a typical shell-shaped or conchoidal surface. Once familiar with its appearance and range of colors, even beginner rockhounds can reliably identify quartz by eye or, if necessary, with a simple scratch test. It is so common in coarse-grained igneous rocks and metamorphic rocks that its absence may be more noteworthy than its presence. And quartz is the main mineral of sand and sandstone. Read more about quartz here.

The uncrystallized version of quartz is called chalcedony ("kal-SED-a-nee"). A hydrated form of silica is called opal, most of which does not resemble the gemstone.


Other Hydrothermal Vein Minerals

Left to right, rose quartz, amethyst and rutilated quartz display some of this mineral's variety.

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Doubly Terminated Quartz Crystal

Dug it myself as a child
Quartz and Silica Minerals Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Double-ended "Herkimer diamond" quartz crystals are found in a few places, but quartz is almost always attached at one end. (more below)

"Herkimer diamonds" are distinctive doubly terminated crystals of quartz from the Cambrian limestones near the town of Herkimer, New York. I dug up this specimen at the Herkimer Diamond Mine as a child, but you can also dig them at the Crystal Grove Mine.

Bubbles and black organic inclusions are common in these crystals. Inclusions make a stone worthless as a gem, but they are valuable scientifically, being samples of the fluids that circulated in the rocks at the time the crystals were being formed.

It's a real thrill to dig for Herkimer diamonds, no matter what age you are. And studying the faces and angles of the crystals will give you an appreciation of their appeal to mystics and to scientists, both of whom take crystal form as a tantalizing clue to the true nature of matter.

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Quartz Spears

The real thing
Quartz and Silica Minerals Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Quartz crystals generally terminate in blades, not true points. Many pointed rock-shop "crystals" are cut and polished quartz.

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Grooves on Quartz Crystal

Look for them
Quartz and Silica Minerals Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

A sure sign of quartz is these grooves across the crystal faces.

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Quartz in Granite

Telltale glitter
Quartz and Silica Minerals Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Quartz (gray) breaks with a conchoidal fracture, making it glitter, whereas feldspar (white) cleaves along crystal planes, making it flash.

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Milky Quartz Clast

Not always shiny
Quartz and Silica Minerals Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Quartz is often milky like this pebble, probably an eroded chunk of a quartz vein. Its tightly interlocked grains do not have the outer form of crystals.

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Rose Quartz

Pink milky quartz
Quartz and Silica Minerals Gallery. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Rose quartz is milky quartz of a pink color, thought to be due to titanium, iron or manganese impurities or microscopic inclusions of other minerals.

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Amethyst

Purple quartz
Quartz and Silica Minerals Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Amethyst, the purple variety of quartz, gets its color from iron atoms in the crystal matrix plus the presence of "holes," where atoms are missing.

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Cairngorm

Brown smoky quartz
Quartz and Silica Minerals Picture Gallery. Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

Cairngorm, named for a Scottish locality, is the dark brown variety of smoky quartz. Its color is due to missing electrons, or holes, plus a whisper of aluminum.

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Quartz in Geode

Two kinds of silica
Quartz and Silica Minerals Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Quartz commonly forms a crust of crystals on the inside of geodes in addition to the layers of chalcedony (cryptocrystalline quartz) in this cut section.

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Chalcedony in a Thunder Egg

A chalcedony core
Quartz and Silica Minerals Gallery. Photo (c) 2003 Andrew Alden, licensed to About.com (fair use policy)

The core of this thunder egg is composed of chalcedony (kal-SED-a-nee), the microcrystalline form of silica. This is about as clear as chalcedony gets. (more below)

Chalcedony is the special name for quartz with microscopically small crystals. Unlike quartz, chalcedony does not look clear and glassy but translucent and waxy; like quartz it is hardness 7 on the Mohs scaleor just a little softer. Unlike quartz it can take on every color imaginable. An even more general term, encompassing quartz, chalcedony and opal, is silica, the compound silicon dioxide (SiO2). Chalcedony may contain a small amount of water.

The major rock type that is defined by the presence of chalcedony is chert. Chalcedony also very commonly occurs as a mineral filling veins and openings, like geodes and this thunder egg.

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Jasper

Authentic poppy jasper
Quartz and Silica Minerals Gallery. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Jasper is a red, iron-rich chert that is rich in chalcedony. Many varieties are named; this is "poppy jasper" from Morgan Hill, California. (click full size)

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Carnelian

Iranian red chalcedony
Quartz and Silica Minerals Gallery. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Carnelian is a red, translucent variety of chalcedony. Its color, like that of jasper, is due to iron impurities. This specimen is from Iran.

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Agate

A gemstone specimen
Quartz and Silica Minerals Gallery. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Agate is a rock (and a gemstone) composed chiefly of chalcedony. This is a particularly refined specimen from Indonesia. (more below)

Agate is the same kind of rock as chert, but in a much purer, more transparent form. It consists of amorphous or cryptocrystalline silica, the mineral chalcedony. Agate forms from solutions of silica at relatively shallow depths and low temperatures, and is exquisitely sensitive to the physical and chemical conditions around it. It is commonly associated with the silica mineral opal. Fossilization, soil formation, and alteration of existing rock can all create agate.

Agate occurs in infinite variety and is a favorite material among lapidaries. Its fluid forms lend themselves to attractive cabochons and similar flat or rounded jewelry formats.

Agate may have several different names, including carnelian, catseye and many fanciful names suggested by the shapes and colors of a particular occurrence.

This stone, magnified several times, displays cracks that extend only a few millimeters from the surface. They are completely healed and do not affect the stone's strength. For a larger specimen, see the agatized tree-trunk in the Fossil Wood Gallery.

For deep geologic information on agates, including hundreds of pictures, visit the Agate Resources page from the University of Nebraska. Agate is the state rock or state gemstone of Florida, Kentucky, Louisiana, Maryland, Minnesota, Montana, Nebraska and North Dakota.

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Cat's-Eye Agate

Chatoyant chalcedony
Quartz and Silica Minerals Gallery. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Microscopic fibers of the amphibole mineral riebekite in this chalcedony specimen produce the optical effect called chatoyancy.

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Opal, Hydrated Silica

A miniature sky
Quartz and Silica Minerals Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Opal combines silica and water in a nearly random molecular structure. Most opal is plain and translucent or milky, but gem opal displays schiller. (more below)

Opal is a delicate mineraloid, hydrated silica or amorphous quartz. The mineral includes a fairly large amount of water molecules, and opals should not be left in direct sunlight or high temperatures.

Opal is a lot more common than people think, but it's usually a thin whitish film that lines fractures in rocks subjected to diagenesis or very mild metamorphism. Opal is commonly found with agate, which is cryptocrystalline quartz. Sometimes it is a bit thicker and has some internal structure that produces the highlights and color range of gem opal. This spectacular example of black opal is from Australia, where nearly all of the world's supply is mined.

The colors of gem opal arise as light diffracts in the ghostly internal structure of the material. The background layer, or potch, behind the colorful part of the opal is important too. The black potch of this black opal makes the colors appear especially strong. More typically, opal has a white potch, translucent potch (crystal opal) or clear potch (jelly opal).

Other Diagenetic Minerals