by John Betterton

The plentiful cryptocrystalline/microcrystalline varieties of quartz are described in this article. Cryptocrystalline/microcrystalline quartz is composed of minute crystals that are made up of twisted, very fine intergrown crystals of quartz with a variable fibre-like morphology. They also contain microscopic pores, fluids and or...

rock n gem magazine issue 70
from rock n gem magazine issue 70

admixed impurities as well. Additional complexity is given by the variable presence of the metastable monoclinic silica polymorph moganite (average 1% to 20%) forming intimate intergrowths within these fibres, which gradually transforms over geological time to quartz. Thus most of the cryptocrystalline/microcrystalline varieties consist of a mixture of quartz and moganite.

These varieties have a hardness of 6.5 to 7 and a specific gravity of around 2.5-2.6, slightly less values compared to the crystalline varieties of quartz. Typically a conchoidal fracture is present in most kinds.
This very large group can be divided into two broad divisions depending on their internal microstructure, namely fibrous and granular groups. We also have the chalcedony name acting partly as a collective term and embraces all the named cryptocrystalline/microcrystalline varieties, and partly as the term applied more strictly to the ordinary generally uniform light coloured, mammillary to botryoidal variety. Several different varieties can be found on a single specimen.

Chalcedony is usually pale in colour with the following shades: almost colourless, grey, white, blue and red with a translucent appearance. It typically is found as irregular rounded, botryoidal, mammillary, stalactitic habits forming crusts and isolated masses. It is often occurs as geodes and amygdule-fillings in basic igneous rocks and in sedimentary rocks. It is very widespread and is found in most countries such as in Argentina, Australia, Austria, Brazil, Bulgaria, Canada, China, Czech Republic, France, Germany, Hungary, India, Italy, Japan, Madagascar, Mexico, Russia, Slovakia, UK, USA, and other locations.
Agate is a common and important sub-variety of chalcedony, and is characterised by distinct banding in which successive layers differ in colour and in degree of translucency. It generally occurs in cavities within basalts, andesites, rhyolites and rhyodacites, as nodules and masses. Less frequent in sedimentary environments. Two main distinctive types of banding are recognised and these are wall-lining (abundant) and horizontal banding (rarer). Fissure filling produces a third type called vein agate. It may be colourless grey white brown, red, orange, black, yellow or multicoloured. The lustre tends to be waxy to dull. Due to the extreme variation in colour and patterns there are a vast number of parietal and trade names for agates. A vast number of locations are known for agates. Only a few can be referred to and include those in Argentina, Austria, Canada, China, Czech Republic, France, India, Italy, Japan, Peru, Russia, Slovakia, South Africa, and the UK. Australia, Brazil, Germany, Mexico and the USA produce the greatest variety of agates.
Onyx is a black and white banded variety of agate and is translucent to opaque with a waxy lustre. A more restrictive distribution in known with sites in Argentina, Australia, Brazil, Canada, China, Czech Republic, Germany, India, Madagascar, Russia, USA producing most of the current specimens.
Carnelian is a sub-variety of chalcedony with a uniformly red to reddish brown, bright orange to deep red colour with all gradations in shade, and it indefinitely passes into sard. It is translucent to opaque with a waxy to resinous lustre. Australia, France, Germany, India, Slovakia, South Africa and the USA are the main sources of good material.
Sard is light to dark brown to brownish-red translucent sub-variety of chalcedony without banding with a waxy to resinous lustre. Good specimens come from scattered localities in Germany, India and the USA.
Sardonyx, reddish-brown and either black or white bands variety of agate that is translucent to opaque with a waxy lustre. Sites within China, Czech Republic, India, Scotland and the USA have supplies collectors with specimens.

Chrysoprase is an apple-green sub-variety of chalcedony and is translucent to opaque. It can be fibrous or microgranular in habit. This variety can be quite brittle and large pieces usually are cracked. Unlike most other chalcedonies, it is the colour rather than the any pattern of markings that makes chrysoprase valuable and the highest desirability. A large number of locations are known for chrysoprase in Australia, Myanmar, Germany, Poland, Russia and especially in the USA. Plasma is a microgranular or microfibrous sub-variety of chalcedony that comes in various shades of green and is nearly always opaque. It may vary due to the presence of white or yellowish patches. India, Madagascar Egypt and the USA have commercial deposits. Bloodstone is dark greenish-blue with small-scattered red blood-like spots and is a translucent to opaque sub-variety of chalcedony. The lustre is waxy to resinous in character. The USA is currently the country with largest number of recorded sites for this variety. It is also known from single locations in Canada, Czech Republic, India, Italy, Romania and South Africa.

The granular group consists of an intergrowth mosaic of microcrystalline quartz, randomly interlocking grains with abundant micropores that form compact, dense and very hard varieties. The fibrous component is ubiquitous but nearly always occurs in minor amounts. These subvarieties of chalcedony include flint, chert and jasper. The name chert is also used as a general group name for fine-grained siliceous sedimentary rocks by sedimentary petrologists. Cherts can be divided into bedded (chert) and nodular (flint) types.

Flint is restricted to the dark opaque dull-coloured chalcedony that occurs as irregular shaped nodules in the Upper Cretaceous Chalk formations. The colour of freshly broken flint is light to dark grey, brownish to black depending on the level of impurities. It is generally darker than chert and usually has a porous surface whitish patina. Flint may show colour banding and has a dull to faintly waxy lustre. A conchoidal fracture is particularly well developed and flint is very tough. Flint can be found in the UK, France, Denmark, Germany, Sweden, Belgium, USA, Spain, Poland, Netherlands and China etc Chert is the name used for chalcedony when it occurs in stratified/bedded or massive form in other rock types, and can be laterally extensive. It is typically greyish, greyish white or pale bluish grey. Less often, it can be dark grey, black, greenish, yellowish, reddish or brownish. Colour mottling also occurs. Chert grades into jasper as the level of impurities increases, particularly the Fe oxides and hydroxides. The lustre is dull chalky to vitreous and translucent on thin edges and opaque in larger masses. It has a conchoidal to splintery fracture. Chert is very widespread across the world with hundreds of localities. A small selection includes those in USA, Canada, UK, China,Australia, India, Japan, Spain, Russia, Saudi Arabia and South Africa.

Jasper is dense, compact, massive, fine–grained chalcedony that contained relatively large amounts of admixed impurities, chiefly Fe oxides. It normally has a deep red to brownish red colour, but also yellow or green. Jasper is commonly variegated, spotted or banded and a uniform appearance is uncommon. The fracture is generally smooth and even, grading to flat-conchoidal in nature. It is mostly opaque and the lustre is dull. Jasper can be found in very extensive beds. It is very common and significant sites are found in USA, Germany, Australia, Austria, Canada, Hungary, China, Italy, Japan, Portugal, Russia, Slovakia, UK, Spain and in other nations.

Addtional polymorphs of silica and other closely related phases will be the topic for the next instalment in the series devoted to the silica minerals.

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