Guide to Quartz
History, Lore and Appreciation
Quartz can be colorless or richly colored, transparent or opaque, highly included or not. It can exhibit chatoyancy, asterism, aventurescence or iridescence. Quartz can be as common as particles of sand on the beach, or deeply coveted gems in private collections and insured at high values. The noted Swiss gemologist, Dr. Eduard Gübelin, aptly referred to quartz as the “jack of all trades.” Yet it is found on every continent on Earth, standing in as one of the world’s most plentiful minerals.
In ancient times, it was believed that transparent colorless quartz was a form of permanent ice, a suggestion first offered by the natural historian, Pliny the Elder. This belief evolved from what was once a major source for quartz, the snow and ice-covered Alps. The word “crystal” in fact derives from this mode of thinking. The Ancient Greek word for ice is kristallos. In 1646, Sir Thomas Browne proved crystal quartz to be a mineral, rather than permanent ice. His book, Pseudodoxia Epidemica, described this as one of his corrections to “vulgar errors.”
Colorless quartz, or rock crystal, has long been cherished for its clarity – references to the clarity of crystal emanate from ancient writings, including the Bible. Some people have long believed that gazing into a large round crystal ball gave clairvoyants an ability to “see” the future.
Colorless quartz with bold and colorful inclusions of another mineral (such as tourmaline, hematite, goethite, mica or rutile) is increasingly used in jewelry. Generally, more colorful quartz varieties, such as amethyst and citrine, are predominantly used in jewelry. (For more about amethyst see page 6; for more about citrine, see page 13).
The transparent brown colored variety of this mineral is called smoky quartz. It was found in the Cairngorm Mountains of Scotland and extensively used in Scottish jewelry in the late nineteenth century. Since then, the majority of brown quartz has been sourced in other localities around the world. The very dark varieties are known in the trade as morion. There is also a small amount of natural green quartz available, called prasiolite, located in the Faroe Islands. However, the majority of the green quartz available on the market today is irradiation-treated quartz also along with some pale amethyst that is heat treated to turn it green.
Rose quartz is a semi-transparent to translucent variety of quartz whose devotees appreciate its soft, pink color. It tends to be very lightly saturated and when cut as a cabochon, it occasionally exhibits asterism with a 6-rayed, sometimes 12-rayed star effect, in direct lighting. Asterism is caused by light reflecting from tiny, oriented, included rutile needles that align themselves in trigonal symmetry during the crystal’s formation. Stars and cat’s eyes occasionally form in smoky quartz and, rarely, in colorless quartz as well.
Aventurine quartzite exhibits phenomena as a result of its inclusions. (Note: Quartzite is a form of quartz; a granular, interlocking mass of quartz crystals formed in different environments, rather than single crystal quartz). Curiously, the term “aventurescence” was so named after a 17th century Italian glassmaker who accidentally tipped copper filings into a batch of molten glass. The result of his fortuitous accident was a glittering form of man-made glass. It is recounted that his colleagues remarked he had made it “per avventura,” or by adventure, or simply, chance. The name stuck. Aventurine glass, which is still manufactured and faceted in Italy, should not be confused with aventurine quartz. Aventurine quartz is green in appearance and its aventurescent effect is generally less pronounced than in glass. Aventurescence in quartzite is due to the granular interlocking of quartz crystals, combined with flat, disc-like inclusions of green mica (fuchsite) that produce glittering reflections in direct light. This optical effect is similar to the one seen in sunstone feldspar due to reflections in the metallic copper (Oregon, USA) or hematite (Tanzania, Africa) inclusions.
Rutilated and tourmalinated quartz (also called sagenitic quartz) contain large, highly visible inclusions that become a celebrated part of the gem itself. Rutile needles may be random, large and golden in color, or may form in multi-rayed, 6-fold star-like inclusions surrounding a hematite crystal within the gem.
Drusy quartz is occasionally used in jewelry design. This is an overgrowth of minute quartz crystals over other larger specimens, or on chalcedony. The result is a glittering, rugged texture that is kept in its rough form and mounted in jewelry.
Birthstones and Anniversaries
Rose quartz is sometimes regarded as an alternate birthstone for the month of January, along with garnet.
Description and Properties
Quartz has a chemical composition of SiO2 and crystallizes in the trigonal crystal system.
Refractive Index: 1.544 to 1.553 Birefringence: 0.009
Specific Gravity: 2.66 (+0.03, -0.02)
Cause(s) of color: (Rose Quartz) Debate about the identity of the nano-inclusions in quartz, which causes both cloudiness and apparent color, remains. In any case, the orientation of the mineral inclusions often causes asterism in rose quartz from Madagascar. (Smoky quartz) Color centers involving aluminum impurities.
Aventurine quartz: Color is caused by inclusions of fuchsite mica platelets.
Mohs Hardness: 7
Internal identifying characteristics:
ln transparent quartz varieties dendritic inclusions, and inclusions of many minerals may be present, including rutile, goethite, schorl (tourmaline), cristobalite, hematite and others. Fluid inclusions can be found in all varieties of transparent quartz.
Quench crackling: Heating, followed by immersion in water causes thermal shock and the gem develops cracks and fissures. This by itself is not attractive, but it is followed by dye impregnation to reach deep inside the quartz through the newly developed, surface-reaching fissures.
The result, for example, can make a gem look surprisingly like emerald or ruby (depending on the dye used).
Irradiation: Colorless quartz can
be irradiated to look smoky, green (prasiolite) or yellow; rose quartz’s color can sometimes be deepened through irradiation.
Heat treatment: Heating may lighten the color of dark smoky quartz.
Coating or foil backing: Deepens the color of some gems; may help with cat’s eye or star phenomena.
Dyeing: (Generally after quench crackling) may cause the material to appear a totally different color.
Rose quartz and other phenomenal varieties are often collected with emphasis placed on depth of color and strength of phenomena. Smoky quartz is collected in antique jewelry – especially Scottish jewelry, due to its historical significance. Artistically carved rock crystal both modern and antique is quite desirable. Inclusions’ connoisseurs collect cut rock crystal with its many mineral inclusions.
Quartz is found on all continents but some stand-out producing countries include: Germany, Hungary, India, Iran, Brazil, Bolivia, South Africa, Madagascar, Mexico, Sri Lanka, Scotland, Spain, Switzerland, and the United States
Cutting, Care and Cleaning
Skilled lapidary artists often carve rock crystal quartz into objects of art, or in unusual shapes. Gems exhibiting phenomena are often cut en cabochon or tablet shapes. While quartz is very durable in general, special care should be taken to not subject it to temperature extremes. Quartz may be cleaned in an ultrasonic cleaner if there are no fissures that could endanger the durability of the stone. As with most gems, using a damp, soft cloth, or gently scrubbing with a soft-bristle toothbrush, is the best way to clean quartz.
Source: CIBJO. Top image: Quartz (var. amethyst), Brooks Davis Collection, Smithsonian.