Quartz

Quartz Crystal is one of the most common Minerals on Earth.  With deposits around the world quartz is one of the most loved gems throughout history in the many forms we all love.  Quartz has always been available in a large variety of color, cuts and at a in-expensive price.

There are two main kinds of crystal quartzMacro-crystalline and Cryptocrystalline Quartz.

There are two main varieties of crystal quartz of the same chemical composition, silicon dioxide, and similar physical properties. Macro-crystalline quartz, includes stones like: Amethyst, Aventurine, rock crystal, blue quartz,  Citrine,  hawk's eye, Prasiolite, quartz cat's eye, smokey quartz,  rose quartz and tiger's eye.

Macrocrystalline, or simply Crystalline, has crystals with distinct shapes recognizable to the naked eye, that run the gamut from tiny druzies all the way up to crystals larger than a man.  



 (Titanium vapor deposition on druzy quartz) 

The photos above are crystalline quartz varieties which include Rock Crystal, Amethyst, Citrine, Ametrine, Prasiolite, Rose Quartz, Blue Quartz, Smokey Quartz, Milky Quartz, Quartzite, and Aventurine, as well as  Hawk's Eye, Tiger's Eye and  Dumortierite quartz, which are usually listed with the crystalline group although some sources list the latter three with the microcrystalline group.  We will include them in the crystalline group.
Colored crystalline quartz varieties generally form as various water solutions are deposited in pegmatite dikes and veins over long periods of time, resulting in slow crystal growth that can yield massive specimens with perfect clarity, that facet beautifully.
Microcrystalline , or Cryptocrystalline quartz has microscopically small crystals that are so minuscule and packed so tightly together that they're completely indistinguishable to the naked eye.  Cryptocrystalline quartz is comprised of the entire Chalcedony group which includes Agates and Jaspers, Chrysoprase, Bloodstone, Carnelian, Onyx, Sardonyx and Sard, and even includes Flint and Chert.

Appearances and colors of quartz vary greatly, but their basic gemological properties remain consistent throughout the entire family, with few exceptions:

Chemical Composition:  SiO2 (silicon dioxide) 

Crystal System:  Trigonal (hexagonal prisms) in many forms like crystalline masses, cryptocrystalline, granular, and in veins.

Mohs:  7 

Specific Gravity: (Density)  2.65 in most forms, but up to 2.91 in Chalcedonies

Refractive Index:  1.544-1.553 (almost always universal in all forms!)

Birefringence:    .009 (some Chalcedonies .004)

Dispersion:  .013 (.008) 

Pleochroism:  None (but can be strong in Rose Quartz, and very weak in Amethyst, Citrine and Ametrine)

Luster:  Vitreous in crystalline varieties; greasy/waxy in cryptocrystalline varieties

Cleavage:  None to indistinct; conchoidal fracture to uneven; brittle, and tough in cryptocrystalline varieties

Optic CharacterUniaxial Positive U+

Optic Sign:  The BULLSEYE interference figure is ONLY seen in quartz, and is therefore DIAGNOSTIC, and easily separates quartz from other gemstones, imitations, or glass; pretty much anything BUT synthetic quartz, which is considered to be impossible for gemologists to detect except in rare cases.


Bullseye interference figure in quartz,
        courtesy of the International School of Gemology 

Luminescence:  Traces of impurities cause wide variance; usually only seen in cryptocrystalline varieties.  White/clear, greens, browns and oranges occasionally fluoresce in LWUV or SWUV (long or short wave ultraviolet light); some material has phosphorescence, and X-rays can cause Rose Quartz to emit a faint blue glow.

Piezoelectricity:  Electric current can be initiated by applying pressure to a quartz slice, and an alternating current applied to a quartz slice can also produce vibration, making it invaluable in communications devices and watches where oscillation is required to produce the desired effect.


Double terminated quartz crystal sandwiched firmly between two crystals in a cluster 

Inclusions in Quartz Crystal:  With 40+ types of mineral inclusions found in crystalline quartz, quartz might just be the most versatile and cost effective gem for studying inclusions.

Cavities in rock crystal are called "negative crystals"; those containing bubbles are known as two-phase inclusions, and interior 'cracks' appear as iridescence.
UFO Quartz Crystal Inclusion

Mineral Inclusions:  Some of the more common mineral inclusions found in crystal quartz:

Actinolite as green fibrous Byssolite

Anatase

Fine particles of Asbestos can cause asterism as a weak six-rayed star in rose quartz, and chatoyancy as cat's eye quartz.

Blue asbestos known as Crocidolite can decompose to form the fibrous blue quartz formation known as Hawk's eye, and when stained with iron oxide becomes the more well-known variety called Tiger's Eye.

Brookite* see red Hematite below

Calcite

Chlorite as green mossy-looking inclusions

Blue-green Chrysocolla

Blue-to-violet Dumortierite

Goethite in yellow to orange wispy fibers or crystals

Gold

Red Hematite platelets


Microphotograph of Hematite or possibly Brookite “Rose” in quartz
courtesy of Conny Forsberg FGA http://vansin.net/ 


Ilmenite 

Mica 

Small droplets of Oil

 Oil “frog” inclusion in quartz crystal courtesy of
the International School of Gemology 


Droplet of oil in crystal quartz
Golden rutile needles in quartz
Sagenite (acicular crystals shaped like needles)

Scapolite

Tourmaline (generally black crystals but other colors exist)
Black tourmaline crystals in quartz

Phantom quartz in crystal quartz
Crystal Quartz can even contain another quartz crystal that formed from a mix with a slightly different chemical makeup than the original quartz, and eventually engulfed it causing a crystal to be visible within the larger crystal; a phenomenon known as a "Phantom" crystal.  The phantom will frequently align with the engulfing crystal as seen above. 
This is by no means a complete list of things that can be found in quartz.  For instance, two of my favorites are pyroxmangite in quartz.
 
Pyroxmangite in quartz

Pink Pyroxmangite appears like floating clouds and gives texture and depth to the stone.…and Medusa quartz which looks like it has tiny blue jellyfish in it:  


"Medusa Quartz", also known as Paraiba quartz, contains gilalite inclusions that can look like medusas rondeau jellyfish floating in the quartz, thus the name.  A naturally occurring quartz, Medusa quartz is a relatively new type of included quartz gemstone that occurs rarely and was only discovered in Brazil in 2004; the gilalite inclusions were identified by the National Museum of Natural History in Paris.

Other structural phenomena occasionally seen in quartz are trigons that look like raised triangles etched onto the surface of prism faces.


Trigons on quartz faces

Due to its piezoelectric properties, hydrothermal synthetic quartz was sought by government agencies who considered it vital to security during WWII.  But producing perfect quartz for use in applications that required extremely pure quartz in order to function properly, such as quartz watches and other industrial applications, remained elusive until the late 60's. 


Rose Quartz crystals:  Normally cut into cabochons, beads, and ornamental gemstone products, rose quartz is associated with love, and is named for its delicate pale-to-deep pink color, which it gets from titanium in the mix it forms from.

Rose quartz rough & heart sculpture      
Faceted rose quartz

Rose quartz  crystal and amethyst crystals are the only two types of colored quartz that do not form large crystals; amethyst forming in geodes, and most rose quartz is found in pegmatites in massive forms that can weigh into the hundreds of pounds.  When rose quartz crystals are rarely seen, they seldom exceed 1cm (½"), and are even more seldom clean enough for faceting; the bulk of material being translucent-to-clouded.

Rose quartz  crystals from certain places like Madagascar contains very fine microscopic-size rutile needle inclusions which, when properly oriented and cut en cabochon, occasionally offers mild asterism that can be discerned as a weak six-rayed star, while in faceted material, the wispy fibers give a cloudy and even silky appearance to the stones.

Rose quartz, like citrine and amethyst, can be subject to fading, both with sunlight or heating above 500ºC.  Sources include Maine, New York and South Dakota in the U.S., as well as Brazil, Africa, India, Japan, and  Russia.


Smoky Quartz crystals :  Caused by either natural or artificial gamma radiation, smoky quartz is the variety named for its smoky colors ranging from pale beige to tan, to light brown, and all the way to dark chocolate colored specimens sometimes called 'moiron', or 'cairngorm' for the Cairngorm Mountains in Scotland where it is found, in addition to other places such as Brazil, Korea, Madagascar, Russia, the Swiss Alps, the Ukraine, as well as California, Pike's Peak in Colorado, and North Carolina in the U.S
Although smoky quartz crystals can also be found in larger sizes, at a certain point they become too dark and opaque to be useful, and Arkansas quartz turns into very dark brown smoky quartz when irradiated, and is mostly sold as specimens.   Irradiated smoky quartz has an unnatural look about it that's easily recognizable for the most part.

Smoky quartz has been sold as smoky topaz to try to command higher prices for the inferior material, and it can also be confused with some common gemstones such as tourmaline and andalusite, as well as some less often seen gems such as axinite, and idocrase.  Inclusions most often seen in smoky quartz are rutile needles.



Quartzite:  Tightly packed grains of quartz formed under high heat and pressure through metamorphism, produces a rock called quartzite which can occasionally contain small inclusions of green aventurine crystals of the chrome-rich mica type known as fuchsite.



Blue Quartz:  A coarse-grained quartzite (aggregate), with crocidolite fiber inclusions that color it blue, deposits of blue quartz come from Virginia in the U.S., Brazil, Austria, South Africa and Scandinavia.  Blue quartz can be easily confused with Dumortierite quartz, a dense deep blue-to-violet variety of crystalline quartz colored by a complex borosilicate known as dumortierite.



Cat's Eye Quartz:  Named for the phenomenon that creates a reaction to light that resembles the reaction seen in cats' eyes, this type of quartz contains inclusions of parallel asbestos fibers or rutile that are all aligned in the same direction, unlike those that cause asterism or stars.  The parallel fibers cause light entering the stone to travel in one direction only, perpendicular to the orientation of the needles, creating the equivalent of a single-rayed star better known as a cat's eye.  The phenomena, called chatoyancy, works much like an aperture, causing the eye to appear to open and close on rotation of the stone.

Sources include Brazil, Sri Lanka, India, Australia and Bavaria, and it can be confused with chrysoberyl cat's eye.  Synthetics are known, and tiger's eye and hawk's eye that has lost its color has been known to be sold as cat's eye quartz.



Aventurine Quartz:  We get the term aventurescence from the reflection of aventurine's minute inclusions of green fuchsite mica in green aventurine, pyrite in brown aventurine, and hematite in reddish-brown aventurine, and the uniform orientation of these inclusions create aventurine's characteristic sparkle.



Always found in massive form, aventurine is cut en cabochon, and is used in ornamental decorations such as sculptures and vases.  It is mined in the Ural Mountains in Siberia, Minas Gerais in Brazil, Austria, Tanzania in Africa, India, Madagascar, Sri Lanka, and in Vermont in the U.S.

Hawk's Eye and Tiger's Eye:   When quartz replaces the blue-to-gray-to-green asbestos fiber known as crocidolite, it forms an opaque aggregate with a silky luster and planes of iridescence known as hawk's eye, so named for the chatoyancy that creates the effect of a bird's eye when light hits it.  Hawk's eye is generally found with it's cousin tiger's eye, which occurs when the iron oxidizes in the crocidolite fibers causing them to turn to shades of brown, while maintaining its fibrous structure which is uneven and crooked, giving it its chatoyant striped appearance and silky luster. 

 Hawk’s Eye

  Tiger’s Eye   
Both are cut en cabochon from slabs that can form a few inches thick, and are found in Africa,  Australia, California in the U.S., Burma (Myanmar), and India.  Dyeing produces red tiger's eye.

1 comment:

  1. that article is good to read...i like that beautiful stone...thanks

    ReplyDelete

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