About the Mineral Quartz
Quartz which is the most common mineral in the Earth's crust after ice and feldspar is found in almost every geological environment and is a component of almost every rock type on Earth. It can be found in sedimentary rocks as grains of sand and as crystals in igneous and metamorphic rocks. Quartzite which is composed almost completely of quartz is produced through the metamorphism of sandstone.
Quartz is the most diverse mineral in terms of varieties, colours and forms because of its abundance and widespread distribution. The name which originates from the old German word "quarz" first appeared in 1530. In Ancient Greece it was known as "krustallos" from "kruos" meaning "icy cold" possibly because of the clarity of some crystals. Theophrastus successor to the Ancient Greek philosopher Aristotle believed this mineral was a unique type of ice that would never melt. The term rock crystal is sometimes used as an alternative name for stones which are exceptionally pure.
This common mineral occurs in two forms, crystalline which is usually abbreviated to crystal (quartz crystal) and cryptocrystalline which is stone made up of crystals so minute they can only be vaguely seen even under high magnification. Although the crystalline varieties of quartz are often colourless and transparent hence the term 'crystal clear', they can also occur in many colours from milky white to dark brown verging on black. These coloured varieties tend to be translucent. Stones that feature crystallized impurities are referred to as quartz with inclusions.
The optical properties of rock crystal led to its extensive use in lenses and prisms and it was also used in electronic components although it has now been replaced with synthetic crystal. Quartz was the first crystal to be used in radio transmission and reception and was essential in the development of computers. On Mohs scale of mineral hardness it grades 7 which means it's a relatively hard stone hence the reason for its popularity as a lapidary material and also for kitchen worktops.
Types of Quartz
Translucent variety of chalcedony that's best known for its distinctive banding
A type of translucent quartz which can usually be identified by the presence of platy inclusions
Reddish to rich orange coloured quartz that's a translucent variety of chalcedony. Sometimes referred to as cornelian
This type of quartz is microcrystalline and white in its purest form. It does however frequently contain inclusions of other minerals which bring about a change in colour. Most types of chalcedony have their own unique name such as agate, bloodstone or carnelian
Translucent apple green variety of chalcedony whose colour comes from the presence of nickel
Yellow to yellowish brown coloured quartz that's relatively rare. Much of the commercial grade citrine that's available to buy is heated amethyst
Inclusions are very common in quartz and although some varieties do have their own name, most are known simply as included quartz. Material that features inclusions of either rutile or tourmaline are probably the best known
A cryptocrystalline type of chalcedony with fine inclusions of varying amounts of other minerals and materials which cause its opacity and colour
Striped variety of agate with alternating black and white bands
A pink coloured stone that's usually translucent, it tends to occur in massive form and crystals are rare. Pink Quartz which is a different type of quartz is often confused for Rose Quartz, the main difference is that crystals are quite common
Colourless and transparent, its name came about during the late Middle Ages in order to differentiate it from a newly produced colourless material known as glass. At that time glass was known as crystal or crystal glass
Translucent to transparent stone that occurs in various shades of brown from light to so dark that it almost appears to be black. Sometimes incorrectly referred to as smoky topaz but topaz is a completely different mineral.
White to grayish white translucent to almost opaque variety of silicon dioxide. Often occurs in the same deposit as rock crystal
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Further ReadingInformation and Links from Wikipedia
Facts and Photos on Geology.com