There are two types of quartz, crystalline quartz and polycrystalline quartz.
This type of quartz is comprised of a single crystal, Rock Crystal is often referred to as simply 'quartz'. The following examples are of crystalline quartz:
Cat's eye quartz
A white, grey, green, yellow or brown chatoyant material, cut en cabochon to produce cat's eye quartz.
A transparent stone with a smoky yellow, brown to black colour caused by natural or artificial radiation.
Rock crystal (quartz)
A clear colourless quartz normally faceted for jewellery.
Rose quartz is pink in colour and when transparent they are faceted.
Stones that have long fine inclusions that run in parallel throughout the stone, produce asterism, They are cut en cabochon to make six-rayed stars.
Rutilated quart displays large radiating needles of rutile in patterns that are often spectacular. Some have whole crystal inclusions at the centre of the radiating mass, they are highly prized collector pieces.
Green quartz is known as prasiolite and vermarine, it is produced by heat-treating amethyst. It is now commonly referred to as green amethyst on the gem market.
Amethyst is a violet to purple variety of crystalline quartz and one of the most popular gemstones for jewellery making. It is often found in geodes from very small to very large sizes, some as large as two metres (and more) have been found, weighing an incredible amount. Smaller geodes are cut in half length-wise to display the intact crystals inside, they are marketed as Amethyst Cathedrals.
A lemon-yellow to golden-yellow variety that occurs naturally, but is mainly produced commercially from heated amethyst. It is also very popular for jewellery making.
Polycrystalline quartz naturally occurs as a mass of microscopic crystals or grains. The grains of some varieties may only be seen with a microscope.
Chalcedony is a mixture of polycrystalline quartz and moganite, a form of monoclinic silica. It has many varieties, one variety is simply called chalcedony. Follow the link for a description of the varieties.
Other varieties of polycrystalline quartz are:
Jasper is an opaque polycrystalline quartz that occurs in red, brown and green colours. The fine grains of jasper make it ideal for carving, resulting in a very smooth finished surface. The patterns forms by the colours present can be very attractive and intriguing. At times only two colours are present, they enable carvings to have, for example, figures mounted on a base where the base is a completely different colour to the figure.
Foosilised wood has been petrified, replaced by minerals over many millions of years. The wood is commonly replaced by jasper chalcedony. It may also be replaced by silicon dioxide, in which case it would have produced opalised wood.
Fossilised wood may be found as large specimens and in many different colours. It is carved into ornamental artefacts and cut into variously shaped cabochons for jewellery making.
Adventurine quartz is a translucent granular material occurring in different colours. The colours are caused by inclusions of differing minerals. Green adventurine quartz is common, the colour is due to an abundance of bright green mica inclusions. The mica inclusions are highly reflective and produce an attractive shimmering effect. Carvings of adventurine quartz are often dyed and sold as a convincing jade simulants.
Tiger's eye typically occurs with alternate layers of yellow-gold and deep red-brown colours. The layers form fine bands and have a long fibrous texture. This produces chatoyancy, a cat's eye effect, together with a very attractive lustre. The gemstone is often used for carvings and cut en cabochon for jewellery making.
Green crystalline quartz (prasiolite or green amethyst) is produced by heat-treating violet amethyst.
Rock crystal may be irradiated to produce smoky quartz.
Synthetic quartz is made (Lab-created) in various colours. Amethyst and citrine may be manufactured in large sized boules, to enable large faceted stones, which are rare and expensive in their natural form.
Polycrystalline quartz is granular and may be dyed to improve the colour or simulate other gem materials.
Quartz Imitations may be made of glass or a dyed material, amethyst and citrine have been imitated in this way.