Amethyst is the birthstone for those who are born in February, while Citrine is a birthstone for November.

Quartz colors
Buying Quartz
Where is Quartz found?
Common Quartz Treatments
World-famous Quartz
Quartz gemology
Quartz legends & lore

Quartz colors
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. Transparency: Crystals are transparent to translucent, cryptocrystalline forms can be translucent or opaque.
The colors of macrocrystalline quartz:

Rock crystal: Colorless. Material that can be cut is rare. Inclusions are of goethite (builds star quartz with rays of light), gold, pyrite, rutile and 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. Most 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. Heat treated violet amethyst or yellowish quartz turns into leek-green prasiolite.

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.

Some of the outstanding chalcedony species if it comes to colors are:

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. Unicolored 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. Cornelian 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.

Buying Quartz
Color
The deep colors are the most valuable.

Lighting
In artificial light Quartz does not display a desirable quality. It looks best in daylight particularly after sunrise and just before sunset.

Clarity
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).

Cut
Due to the roughness of the color distribution in the crystals, quartz is often cut as brilliant round cut to maximize the color. Other cuts can be used when the color is better distributed. Quartz is available in a wide range of calibrated sizes and shapes, including many fancy shapes.

Quartz location and deposits
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

Cryptocrystalline quartz deposits:
Australia: Agate, chrysoprase, bloodstone
Brazil: Agate, chalcedony, carnelian, bloodstone, chrysoprase
China: Agate, bloodstone
India: Agate, chalcedony, bloodstone, carnelian, chrysoprase
Kazakhstan: Chrysoprase
Madagascar: Agate, chalcedony, chrysoprase
Mexico: Agate
Mongolia: Agate
Namibia: Agate, blue chalcedony, chalcedony,
Russia: Chrysoprase
Sri Lanka: Chalcedony,
Uruguay: Agate, chalcedony, carnelian
South Africa: Chrysoprase
Tanzania: chrysoprase
Zimbabwe: Chalcedony, chrysoprase
USA: Agate (Montana and Wyoming), chalcedony (California), chrysocolla chalcedony (Arizona), bloodstone, blue chalcedony (California, Nevada, Oregon)

Common Quartz treatments
Colorless quartz is always untreated. Colored stones can occasionally be enhanced in color by dying (a combination of bathing in color agent solutions, fallowed by heating), irradiation (bombardment with low level radioactivity), or heating. Reliable gem dealers will always inform their customers about any kind of treatment.
Some quartz gems can lose their color saturation over a period of time in daylight. The original color can be restored by X-ray radiation.

World-famous Quartz
Chrysoprase, the bright apple green translucent chalcedony, was a particular favorite of Frederick The Great of Prussia. It can be seen today decorating many buildings in beautiful Prague, including the Chapel of St Wencelas.

Fine amethysts are featured in the British Crown Jewels and were also a favorite of Catherine the Great and Egyptian royalty. In former days amethyst was a favorite stone in the high ranks of the Christian church, therefore it was called "the stone of bishops".

One of the finest known rock crystal pieces is the 12.75 inch diameter, 107 pound flawless "crystal ball" in the collection of the Smithsonian Institution in Washington D.C.

Quartz is one of the first gems to be synthetically grown on a large scale. Major development was done during World War II, to supply crystals for radios. Today, our computer industry is based on synthetic quartz.

Quartz gemology
Species: Macrocrystalline Quartz
Chemical composition: SiO, silicon dioxide
Hardness: 7 (Mohs scale)
Specific gravity: 2.65
Refractive index: 1.544 - 1.553
Birefringence: +0.009
Color of streak: White
Absorption spectrum, color, crystal system and fluorescence differ within the varieties.

Quartz gemology
Species: Cryptocrystalline Quartz
Chemical composition: SiO, silicon dioxide
Hardness: 6.5-7 (Mohs scale)
Specific gravity: 2.58 - 2.64
Refractive index: 1.530 - 1.540
Birefringence: up to 0.004
Color of streak: White
Absorption spectrum, color, crystal system and fluorescence differ within the varieties.

The Quartz zodiac, myth & legend
Leonardo da Vinci wrote that amethyst, the most valuable stone in the quartz family, was able to dissipate evil thoughts and quicken the intelligence.

Rock crystal and smoky quartz were once used for crystal balls that disclosed fortunetellers, witches or gypsy grandmas the future of their clients.

Amethyst is the birthstone for those who are born in February, while Citrine is a birthstone for November.

In Antiquity, as well as in the Middle Ages people believed that the cosmos is reflected in gemstones. The amethyst is assigned to planet Neptune. Citrine is assigned to planet Mercury as well as tiger's eye. Rose quartz is assigned to planet Venus. Chalcedony is assigned to Planet Saturn. Smoky quartz is assigned to planet Pluto. The esoteric movement revived the ancient belief and the gem industry made it another marketing tool to promote certain gems.

The healing powers of gems remain a controversial issue, but are mentioned for centuries by healers, shamans and medicine men. Whether it's a fact or a placebo effect doesn't matter, if it helps. The safest approach is to wear the gemstone in skin contact to the troubled part of the body. Quartz is said to be of great helping potential.
Headaches: Amethyst, rock crystal, tiger's eye
Eye inflammation: Rock crystal, chalcedony, hawk's eye, onyx
Sore throat: Rock crystal
Heart ailments: Rose chalcedony, rose quartz
Pancreas: Amethyst, citrine, yellow jasper
Sciatica: Amethyst, citrine, rose quartz, tiger's eye
Varicose veins: Rock crystal
Testicles: Rock crystal, chalcedony, rose quartz, tiger's eye
Toenails: Rose quartz