Back to Rocks and Minerals Articles

dark grey quartz crystal

Smoky quartz. Arkansas, U.S.A. Johann Werner Collection, Earth Sciences Museum.

Quartz is our most common mineral. Quartz is made of the two most abundant chemical elements on Earth: oxygen and silicon. Atoms of oxygen and silicon join together as tetrahedrons (three sided pyramids). These stack together to build crystals. Billions of tetrahedrons are needed to build even a small crystal. Quartz is an almost pure chemical compound with constant physical properties.

Quartz makes up about 12 percent of the land surface and about 20 percent of the Earth's crust. Most of the remaining crustal rocks are rich in silicate minerals which include silicon and oxygen together with other elements.

Quartz is divided into two groups; crystalline or visible crystals and cryptocrystalline or hidden crystals which, require a microscope.

Crystalline quartz

Crystalline quartz is found as tiny crystal frostings sparkling on a rock surface as well as well-formed crystals weighing tonnes. Quartz crystals are named from the colour of the crystals:

Purple- amethyst

Pink– rose- tiny amounts of titanium oxide crystals (rutile) give it the pink colour

Clear- rock crystal (originally thought to be frozen water)

White- Milky Quartz– tiny bubbles in the crystals give it the milky appearance

Yellow, brown and black– smoky quartz– natural radiation in the rocks causes colouration

Crystalline quartz has a hardness of 7.0 making it suitable for use as a gemstone.

Crystalline quartz is the most common weathering-resistant mineral. This explains its presence as the main ingredient in beach sand. In some parts of the world beach sand has a thin layer of silica gel on the outside. This causes the sand to squeak when walked on.

Birds migrating from Canada to warmer climates in the fall take with them quartz sand grains in their crops. These sand grains are found in areas where no other deposits of sand are to be found.

Cryptocrystalline quartz

Chalcedony or cryptocrystalline quartz does not form visible crystals.

Banded agate is formed by the deposition of silicon dioxide from a hot water solution. Tiny amounts of iron and other impurities give it a variety of colours. Most of the bright colours of agate (pink, green, blue) are formed by dyeing grey and white banded agate. Some brown agates are coloured by heating. Some agate preserves petrified wood and dinosaur bone. Even the microscopic cells are replaced so that the original tree can be identified or dinosaur bone structure studied.

Flint and chert are examples of cryptocrystalline quartz without banding. These are usually found as gray, black or white nodules in limestone and dolostone. Silica dissolved and precipitated from sponge spicules and microscopic plants such as diatoms form flint and chert. For centuries this material was chipped into arrowheads and other tools.

Jaspers are usually yellow, orange and brown due to the presence of iron.

Chrysoprase is coloured green by the presence of nickel.

brown, white and grey banding in agate specimen


Opal is silicon dioxide with additional water. The rainbow colours of opal are formed by layers of tiny opal spheres which interfere with the light in the same way that an oil film floating on water causes colours to appear. Opal is deposited by hot springs, or by low temperature water. The largest accumulations of opal are the siliceous skeletons of silica secreting organisms such as diatoms and grasses.

Products made from quartz


Silicon is used in the steel industry as a constituent of silicon-steel alloys. Silicon is also used as an alloy in copper, brass and bronze. Silicon is a semiconductor, in which the resistivity to the flow of electricity at room temperature ranges between that of metals and that of insulators. The conductivity of silicon can be controlled by adding small amounts of impurities, called dopants. The ability to control the electrical properties of silicon, and its abundance in nature, have made possible the development and widespread application of transistors and integrated circuits used in the electronics industry. Silica and silicates are used in the manufacture of glass, glazes, enamels, cement and porcelain, and have important individual applications. Fused silica, a glass made by melting quartz or hydrolyzing silicon tetrachloride, has a low coefficient of expansion and high resistance to most other chemicals. Silica gel is a colourless, porous, amorphous substance. It is prepared by removing part of the water from a gelatinous precipitate of silicic acid, which is formed by adding hydrochloric acid to a solution of sodium silicate. Silica gel absorbs water and other substances and is used as a drying and decolourizing agent.

  • Anti-caking powder used in foods (check the label for silicon dioxide) Coffee Creamers use sodium aluminum silicate (feldspar) for the same job!
  • Bricks
  • Cement
  • Ceramics
  • Cooking stoves with halogen heating elements
  • Fibreglass, used for insulation and for building boats.
  • Glass
  • Gemstones
  • Halogen lamps (quartz glass)
  • Oscillators - for radio, watches etc.
  • Pyrex heat resistant glass and quartz glassware for
  • Scientific use

Silicon metal

Silicon metal is produced by the removal of the oxygen from the quartz. Raw silicon metal is grown on a seed crystal in a furnace at 1500¼C. Crystals 15 to 20 cm in diameter can be produced in this way. Slices of this crystal are cut, polished and photo etched to create computer chips and solar cells.


Silicones combine organic materials such as oil, rubber and plastic with silicon.

  • Adhesive tapes and sheets for wave soldering circuit boards Casting material for leaves and other organic specimens for museum displays
  • Conductive greases and rubber compounds - with silver etc
  • Conveyor belts
  • Cover for high voltage cables
  • Gel sheets to reduce scaring from wounds
  • Hiking boots - leather silicone impregnated
  • Implants
  • Insoles for sports shoes
  • Lipstick
  • Light pipes and sensors for organic compounds
  • Netting for skin grafts
  • Oil - used in retinal detachment surgery
  • Pads for silk screen printing
  • Prosthetic devices for amputees
  • Sealants to insulate windows, bathtubs etc
  • Silicone washed denim for jeans
  • Silicone Rubber
  • Skin Lotion
  • Swim goggles - scuba masks
  • Thermal and acoustic insulation
  • Transformer oil
  • Water Repellents for concrete and masonry
  • Windshield wipers