Quartz crystals: Herkimer “Diamond” quartz crystals. A clear, “rock crystal” variety of quartz.
What is Quartz?
Quartz is a chemical compound consisting of one part silicon and two parts oxygen. It is silicon dioxide (SiO2). It is the most
abundant mineral found at Earth’s surface, and its unique properties make it one
of the most useful natural substances.
Rock crystal quartz: Transparent “rock crystal” quartz. This specimen shows the conchoidal fracture (fracture that produces curved surfaces) that is characteristic of the mineral. Specimen is about four inches (ten centimeters) across and is from Minas Gerais, Brazil.
Where is Quartz Found?
Quartz is the most abundant and widely distributed mineral found at Earth’s
surface. It is present and plentiful in all parts of the world. It forms at
all temperatures. It is abundant in igneous, metamorphic, and sedimentary
rocks. It is highly resistant to both mechanical and chemical weathering.
This durability makes it the dominant mineral of mountaintops and the primary
constituent of beach, river, and desert sand. Quartz is ubiquitous, plentiful
and durable. Minable deposits are found throughout the world.
Amethyst quartz: Purple crystalline quartz is known as “amethyst.” When transparent and of high quality, it is often cut as a gemstone. This specimen is about four inches (ten centimeters) across and is from Guanajuato, Mexico.
Physical Properties of Quartz
|Color||Quartz occurs in virtually every color. Common colors are clear, white, gray, purple, yellow, brown, black, pink, green, red.|
|Streak||Colorless (harder than the streak plate)|
|Diaphaneity||Transparent to translucent|
|Cleavage||None – typically breaks with a conchoidal fracture|
|Specific Gravity||2.6 to 2.7|
|Diagnostic Properties||Conchoidal fracture, glassy luster, hardness|
|Uses||Glass making, abrasive, foundry sand, hydraulic fracturing proppant, gemstones|
Flint: Flint is a variety of microcrystalline or cryptocrystalline quartz. It occurs as nodules and concretionary masses and less frequently as a layered deposit. It breaks consistently with a conchoidal fracture and was one of the first materials used to make tools by early people. They used it to make cutting tools. After thousands of years, people continue to use it. It is presently used as the cutting edge in some of the finest surgical tools. This specimen is about four inches (ten centimeters) across and is from Dover Cliffs, England.
Quartz flint arrowheads: One of the first uses of quartz, in the form of flint, was the production of sharp objects such as knife blades, scrapers, and projectile points such as the arrowheads shown above. Image copyright iStockphoto / Leslie Banks.
What are the Uses for Quartz?
Quartz is one of the most useful natural materials. Its usefulness can be
linked to its physical and chemical properties. It has a hardness of seven
on the Mohs Scale which makes it very durable. It is chemically inert in
contact with most substances. It has electrical properties and heat resistance that make it valuable in electronic products. Its luster, color, and
diaphaneity make it useful as a gemstone and also in the making of glass.
Uses of Quartz in Glass Making
Geological processes have occasionally deposited sands that are composed of almost 100%
quartz grains. These deposits have been identified and produced as sources of high purity
silica sand. These sands are used in the glassmaking industry.
Quartz sand is used in the production of container glass, flat plate glass, specialty glass, and fiberglass.
Quartz glass windows: Glassmaking is one of the primary uses of quartz. Image copyright iStockphoto / Chinaface.
Jasper beads: Quartz is often used in jewelry or as a gemstone. These jasper beads are an example of quartz used as a gemstone.
Quartz glass sand: High-purity quartz sandstone suitable for the manufacture of high-quality glass. “Glass sand” is a sandstone that is composed almost entirely of quartz grains. Pictured here is a specimen of the Oriskany Sandstone from Hancock, West Virginia. In a few locations, the Oriskany is over 99% pure quartz. Much of it has been used for container glass, but some of it has been selected for use in making lenses for the largest telescopes. Specimen is about four inches (ten centimeters across).
Blue aventurine quartz: Aventurine is colorful variety of quartz that contains abundant shiny inclusions of minerals such as mica or hematite. It is often cut and polished for use as an ornamental stone. Common colors for aventurine are green, orange, and blue. This specimen is about four inches (ten centimeters) across and is from India.
Uses of Quartz as an Abrasive
The high hardness of quartz, seven on the Mohs Scale, makes it harder than most other
natural substances. As such it is an excellent abrasive material. Quartz sands and
finely ground silica sand are used for sand blasting, scouring cleansers, grinding
media, and grit for sanding and sawing.
Uses of Quartz as a Foundry Sand
Quartz is very resistant to both chemicals and heat. It is therefore often used
as a foundry sand. With a melting temperature higher than most metals, it can
be used for the molds and cores of common foundry work. Refractory bricks are often made of quartz sand because of its high heat resistance. Quartz sand is also used as a flux in the smelting of metals.
Silicified wood: Silicified “petrified” wood is formed when buried plant debris is infiltrated with mineral-bearing waters which precipitate quartz. This quartz infills the cavities within the wood and often replaces the woody tissues. This specimen is about four inches (ten centimeters) across and is from Yuma County, Arizona.
Uses in the Petroleum Industry
Quartz sand has a high resistance to being crushed. In the petroleum industry, sand slurries are forced down oil and gas wells under very high pressures in a process known as hydraulic fracturing. This high pressure fractures the reservoir rocks, and the sandy slurry injects into the fractures. The durable sand grains hold the fractures open after the pressure is released. These open fractures facilitate the flow of natural gas into the well bore.
The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology.com Store.
Chert: Chert is a microcrystalline or cryptocrystalline quartz. It occurs as nodules and concretionary masses and less frequently as a layered deposit. This specimen is about four inches (ten centimeters) across and is from Joplin, Missouri.
Many Other Quartz Sand Uses
Quartz sand is used as a filler in the manufacture of rubber, paint, and putty. Screened and washed, carefully sized quartz grains are used as filter media and roofing granules. Quartz sands are used for traction in the railroad and mining industries. These sands are also used in recreation on golf courses, volleyball courts, baseball fields, children’s sand boxes and beaches.
Quartz crystal: A Herkimer “Diamond” quartz crystal in dolostone. This specimen is about six inches (fifteen centimeters) across and is from Middleville, New York.
Uses for Quartz Crystals
One of the most amazing properties of quartz is the ability of its crystals to vibrate at a precise frequencies. These frequencies are so precise that quartz crystals can be used to make extremely accurate time-keeping instruments and equipment that can transmit radio and television signals with precise and stable frequencies.
The tiny devices used for these purposes are known as “crystal oscillators.” The first crystal oscillators were developed in the 1920s, and just twenty years later, tens of millions of them were needed each year to supply the military during World War II. Today, billions of quartz crystals are used to make oscillators for watches, clocks, radios, televisions, electronic games, computers, cell phones, electronic meters, and GPS equipment.
A wide variety of uses have also been developed for optical-grade quartz crystals. They are used to make specialized lenses, windows and filters used in lasers, microscopes, telescopes, electronic sensors, and scientific instruments. The material of beach sand is now the material of the world’s most advanced electronic devices.
A Need for Synthetic Quartz Crystals
During the 1900s the demand for high-quality quartz crystals accelerated so rapidly that mining operations around the world were unable to supply them in adequate quantities. Fortunately, this need was realized during World War II, and military and private industry began working on methods to grow synthetic quartz crystals to meet the special requirements of optical and electronics use.
Today, most of the quartz crystals used in electronic components and optical instruments are grown in laboratories instead of produced from mines. Most of the laboratories grow their crystals using methods based upon the geological process of hydrothermal activity. The synthetic crystals are grown at high temperatures from superheated waters that are rich in dissolved silica. These manufactured crystals can be grown in shapes, sizes and colors that match the needs of manufacturing processes. The cost of growing synthetic quartz crystals is competitive with mining, and the only limit on production is the availability of crystal growth equipment.
Ametrine: A bicolor stone combining golden citrine and purple amethyst. This gem measures about 8×10 mm.
Quartz as a Gemstone
Quartz makes an excellent gemstone. It is hard, durable, and usually accepts a brilliant polish. Popular varieties of quartz that are widely used as gems include: amethyst, citrine, rose quartz, smoky quartz, and aventurine. Agate and jasper are also varieties of quartz with a microcrystalline structure.
Rose quartz beads: Translucent rose quartz – cut and polished beads. Each bead is about ten millimeters in diameter.
Novaculite is a dense, cryptocrystalline variety of quartz with a fine-grained and very uniform texture. As quartz, it has a hardness of 7 (harder than steel) and is used as a “whetstone” for sharpening knives.
Special Silica Stone Uses
“Silica stone” is an industrial term for materials such as quartzite, novaculite, and other microcrystalline quartz rocks. These are used to produce abrasive tools, deburring media, grinding stones, hones, oilstones, stone files, tube-mill liners, and whetstones.
Tripoli is crystalline silica of an extremely fine grain size (less than ten micrometers). Commercial tripoli is a nearly pure silica material that is used for a variety of mild abrasive purposes which include: soaps, toothpastes, metal-polishing compounds, jewelry-polishing compounds, and buffing compounds. It can be used as a polish when making tumbled stones in a rock tumbler. Tripoli is also used in brake friction products, fillers in enamel, caulking compounds, plastic, paint, rubber, and refractories.
Find Other Topics on Geology.com: