QUARTZ Gemology NOTES
PUBLISHED BY MARCEL VOGEL AND P.R.I.
Species: Macrocrystalline Quartz
Chemical composition: SiO2, silicon dioxide
Specific gravity: 2.65
Refractive index: 1.544 – 1.553
Birefringence: up to 0.009
Color of streak: White
Absorption spectrum, color, crystal system and fluorescence differ within the varieties.
Quartz is not a renewable resource. Certain Quartz Crystal specimens have high value
Quartz is one of the most common minerals on earth and is well known in the gems world. Quartz
is attractive and durable, as well as scientifically and spiritually invaluable. It can be cut and carved in many forms and sizes.
There are two main varieties of quartz, though they share the same chemical composition, silicon dioxide. Macrocrystalline quartz, includes stones like amethyst, aventurine, rock crystal, blue quartz, citrine, hawk’s eye, prasiolite, quartz cat’s eye, smoky quartz, rose quartz and tiger’s eye. The quartz is mostly transparent to translucent. Cryptocrystalline quartz, with microscopically small crystals, is known as chalcedony, and includes agate, chrysoprase, bloodstone, jasper and carnelian. Crytocrystalline quartz is usually opaque or translucent.
The color of macrocrystalline quartz is as variable as the spectrum, but clear quartz is by far the most common color followed by white or cloudy. Purple (amethyst), pink (rose quartz), gray or brown to black (smoky quartz) are also common. Cryptocrystalline quartz varieties can be multicolored. Luster is glassy to vitreous as crystals, while cryptocrystalline forms are usually waxy to dull but can be vitreous. Crystals are transparent to translucent; cryptocrystalline forms are usually translucent or opaque.
macrocrystalline quartz :
Rock crystal: Colorless. Material that can be cut is rare. Inclusions are of goethite, gold, pyrite, rutile or tourmaline. The luster is vitreous.
Smoky quartz: Brown to black, smoky gray. The coloring is caused by natural and artificial gamma rays.Frequent inclusions are rutile needles.
Amethyst: Purple, violet, pale red-violet. Amethyst is the most highly valued stone in the quartz group. The coloring agent is iron.
Amethyst Quartz: Violet with whitish stripes. Amethyst quartz is a more compact formation of amethyst, layered and striped with milky quartz.
Ametrine: Yellow and violet. Color-zoned quartz variety that consists half of amethyst and citrine.
Citrine: Light yellow to dark yellow, gold-brown. The coloring agent is iron. Many commercial citrines are heat-treated amethyst. Natural citrines are mostly pale yellow. If heat-treated they acquire a reddish tint.
Prasiolite: Leek-green. Prasiolite is not found in nature. Prasiolite is produced by heat treatment of amethyst or yellowish quartz.
Rose quartz: Strong pink, pale pink. Coloring agent is titanium. Traces of included rutile needles cause six-rayed stars when cut en cabochon. Larger clear stones can be faceted. Rose quartz crystals tend to be cloudy, which deepens the color. Transparent crystals are extremely rare.
Aventurine: Green, red-brown, gold-brown. Mostly dark green with metallic glittery appearance caused by included fuchsite (green mica), or red- to gold-brown caused by hematite leaves.
Prase: Leek-green. Prase is a quartz aggregate, usually classified as a chalcedony whose color agent are chlorite inclusions.
Blue quartz: Turbid blue. Inclusions of crocidolite fibers cause the color.
Quartz cat’s eye: White, gray, green, yellow, brown (Cat’s eye: Is a reflection of light by parallel fibers, needles, or channels, which resembles the slit eye of a cat. When the stone is rotated, the cat’s eye glides over the surface.)
Hawk’s eye: Blue-gray to blue green (Hawk’s eye: Small ray of light on the surface that is reminiscent of the eye of a bird of prey)
Tiger’s eye: Gold-yellow, gold-brown (Similar to Hawk’s eye effect, ray of light is brown colored due to oxidized iron inclusions)
The colors of cryptocrystalline quartz:
In the narrow sense, chalcedony (species name for all cryptocrystalline quartzes) will mean any translucent, cryptocrystalline quartz with a single color, whether it has a special variety name or not. Its colors are bluish,
white, or gray. The various types differ in color due to metallic impurities, such as iron, nickel, copper, and titanium, present during crystallization.
Agate is distinguished by having multiple colors. Banded agates are some of the most popular. A rarity is the so-called fire agate. Fire agate: The iridescent colors of red, gold, green and rarely, blue-violet, result from interference between light rays traveling through these thin layers. Agate jasper, which grows together with agate, is yellow, brown, or green blended. Agate comes in many different trade names. The most important ones are: Dendritic agate: Colorless or whitish, translucent chalcedony with tree- or fern-like markings (called dendrites). Moss agate: Colorless with green, brown or red inclusions. Moss agate is a translucent chalcedony with moss-like inclusions of hornblende or chlorite. Scenic agate: Agate where the included dedrites resemble landscape-like images in brown or reddish color tones. Onyx is a layered stone with a black base and a white upper layer.
Bloodstone is an opaque, dark-green chalcedony with red spots (caused by iron oxide).
Blue chalcedony, called “Mohave” and “Mt. Airy Blues”, originate in California and Nevada, are slightly to moderately grayish blue with a light to medium color range. Blue chalcedony from Namibia, often called “African Blue”, varies from grayish to nearly pure blue and from light to medium dark. The most unusual and most valuable type is from Oregon. Its blues are modified by slight to moderate amounts of pink, making a noticeably lavender gem, which nonetheless is called “Holly Blue.”
Chalcedony, in the narrow sense, comes in bluish white or gray. Uni-colored chalcedony sometimes is called onyx.
Carnelian ranges in color from yellow-orange to rich, near reddish orange, to orangey brown, and varies from semi-opaque to highly translucent. The color agent is iron. The color can be enhanced by heating. Carnelian onyx is a layer stone with a red base and a white upper layer.
Chrysoprase, apple green chalcedony that derives its color from nickel, is ranging from nearly opaque to nearly transparent. Its color spectrum includes olivey, to nearly pure greens of medium tone. Very fine, highly saturated pieces have been successfully misrepresented as Imperial jade.
Chrysocolla Chalcedony, marketed as “Gem Silica” this relatively rare, blue to blue-green, opaque to near transparent material is the most expensive type of chalcedony. Its color agent is copper.
A fine Quartz is transparent, which means, the light passes the stone unhindered. A translucent quartz slightly weakens the passage of the light through the stone. The best quality quartz is “clean”, free of inclusions of any kind. As the stone is plentiful, there is little reason to go for stones with visible inclusions, except those that define the character of the stone (e.g. Cat’s eye, Hawk’s eye or scenic stones).
Quartz is one of the most common minerals on earth.
Macrocrystalline quartz deposits:
Rock crystal: In the Alps, Brazil, Madagascar, United States
Smoky quartz: Brazil, Madagascar, Russia, Scotland, Switzerland, Ukraine
Amethyst: Brazil, Bolivia, Canada, India, Madagascar, Mexico, Myanmar (Burma), Namibia, Russia, Sri Lanka, United States (Arizona), Uruguay and Zambia
Amethyst quartz: Brazil, Bolivia, Canada, India, Madagascar, Mexico, Myanmar (Burma), Namibia, Russia, Sri Lanka, United States (Arizona), Uruguay and Zambia
Ametrine: Brazil, Bolivia
Citrine: Argentina, Brazil, Madagascar, Namibia, Russia, Scotland, Spain, United States
Prasiolite: Brazil, United States (Arizona)
Rose quartz: Brazil, India, Madagascar, Mozambique, Namibia, Sri Lanka, USA
Aventurine: Austria, Brazil, India, Russia, Tanzania
Prase: Austria, Finland, Germany, Scotland
Quartz cat’s eye: Brazil, India, Sri Lanka
Hawk’s eye: Brazil, India, Sri Lanka
Tiger’s eye: Australia, India, Myanmar, Namibia, South Africa, Sri Lanka, United States
Common Quartz TREATMENTS
Colorless quartz is always untreated. Colored stones can occasionally be enhanced in color by dying (as in the case of agate), irradiation (bombardment with low level radioactivity), or heating. Reliable gem dealers will always inform their customers about any kind of treatment.