Quartz Once Believed to be a Unique Type of Ice
Quartz which is the most common mineral in the earth's crust after ice and feldspar is found in almost every geological environment. A component of virtually 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 quartz was known as 'krustallos' from 'kruos' meaning 'icy cold' possibly because of the clarity of some crystals. Theophrastus successor to Aristotle believed it was a unique type of ice that would never melt.
Quartz crystal cluster (Peru). Photo; Stan Celestian. Clickable image
The name rock crystal is sometimes used but only applies to very pure varieties of quartz. In its purest form this mineral is colourless and transparent.
Quartz occurs in two forms, crystalline which is usually abbreviated to crystal (quartz crystal) and cryptocrystalline which means its crystals are so minute they're too small to be seen with the naked eye. Although crystalline varieties are often colourless and transparent hence the term 'crystal clear', they also occur in many different colours from milky white to dark brown verging on black. The coloured varieties of quartz tend to be translucent as opposed to transparent. Stones featuring crystallized impurities are known as quartz with inclusions or included quartz.
The optical properties in quartz crystal led to its extensive use in lenses and prisms. It was once widely used in electronic components although has now been replaced with a synthetic alternative. Quartz was the first crystal to be used in radio transmission and was essential in the development of computers. When mechanical pressure is applied to a quartz crystal it returns an electrical charge.
Pure crystalline quartz surrounded by the mineral sodalite. Photo; Stone Mania. Clickable image
On Mohs scale of mineral hardness quartz grades 7 which makes it a relatively hard material. It's important to note that hardness refers to a stone's resistance to being scratched and is not the same as toughness. Tenacity which is the correct geological term describes a rock or mineral's resistance to being broken, crushed, bent or misshapen. Despite being quite hard quartz is brittle so will break or shatter easily.
Different Quartz Varieties
Translucent variety of chalcedony 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, 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
A combination of colourless or milky white quartz combined with amethyst. Named for its chevron-like markings although in the UK this mineral tends to be known as banded amethyst.
Translucent apple green variety of chalcedony whose colour comes from the presence of nickel
Inclusions are common in quartz and although some varieties have their own name, most are known simply as included quartz. Tourmalinated and rutilated quartz are probably the best known varieties of included quartz
A cryptocrystalline variety of chalcedony with fine inclusions of varying amounts of other minerals and substances which are responsible for the stone's 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 considered to be 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 mineral 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 to quartz.