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Buying Guide: Get to Know the Different Types of Chalcedony



Quartz gemstones provide the jewellery trade with more varieties than any other gem species. Here, Gem-A Tutor Pat Daly FGA DGA explores one of the subdivisions of quartz – chalcedony - and discovers the stones (and challenges) that new and experienced gemmologists will discover in the field. 

Some quartz varieties, such as amethyst, citrine, and rose quartz, are cut from single crystals, but more are polycrystalline. This means that a stone is composed of many interlocking crystals. Some polycrystalline quartz gems have individual crystals which are large enough to be seen with a loupe, but those of others may be so small that they are not visible through an optical microscope. Problems of terminology begin here. 

Microcrystalline or Cryptocrystalline?

Some authors state that chalcedony is microcrystalline, meaning that crystals can be seen with a microscope, others that it is cryptocrystalline, and that they cannot, and some use both terms. In any case, these definitions of micro- and cryptocrystalline depend on the type of microscope used and the way in which the stone has been prepared. The lesson for gemmologists is that hard and fast boundaries do not exist in nature or in many aspects of the gem trade. Where differences between gem varieties are concerned, this ought to be borne in mind.

Chalcedony polished stones and cabochons, photographed by Pat Daly.

What is Chalcedony?

It is generally agreed that chalcedony is a quartz gem composed of very small crystals which cannot be distinguished with a loupe or a standard gemmological microscope. Most types, such as chalcedony, agate, and jasper are included in this description but coarser grained varieties, such as aventurine and tiger’s eye are not. 

Chalcedony specimen from the Gem-A Archives, photographed by Henry Mesa.


Types of Chalcedony: Agate

Agate is a type of chalcedony which occurs as nodules, often in spaces which were once bubbles in volcanic rocks. It grows inwards from the walls of the cavities as fibrous crystals of quartz and often, though not always, it is marked by concentric colour bands. The fibres radiate out from nucleation points, forming a series of domes which compete for space as they grow. Structural banding, parallel with the advancing growth surfaces, is always present even if colour banding is absent. 

Pink chalcedony from the Gem-A Archives, photographed by Henry Mesa. 


A slice of translucent agate held a little way in front of a torch or light bulb shows the fibrous structure, concentric banding, and the boundaries between the dome-like masses of radiating fibres. In a stone cut from an agate, the fibres and banding may not be evident, but it is usually possible to see these boundaries. Some material which grows in this way would be called chalcedony in the gem trade because the term is often used for stones of uniform colour. 

Colour Banding in Agates

Colour banding in agates may be curved and concentric, or straight. Agates which have complete concentric banding are called fortification agates because of their resemblance to the ramparts of hill forts. Straight-banded agates are called onyx. They are useful for cutting cameos and intaglios, tabular gems with carvings on one side. Any colour may alternate with white, but the name onyx is used mostly for black and white-banded stones. Nowadays, the name onyx is often given to uniformly black stones. The objection that this material should be called dyed black chalcedony is often disregarded. 

Agate with concentric bands photographed by Pat Daly.


Many agates from Brazil, which is the main commercial supplier, are grey to brownish, and unsuitable for jewellery. They may be dyed to any colour, however, and both uniform and banded types are common. 

Favourably oriented agates may present the image of a landscape, small concentric features resembling eyes, or features resembling candle flames. Fine examples of landscape agates are valuable stones. Crystals may grow into cavities which are later filled with agate. They are usually replaced by agate and form sprays or sunburst patterns within the stones, which are called sagenitic agates. Agates may be named after a locality where they are found, but there are so many of these that a keen interest in the stones is required to remember them.

Iridescence in Agates

Iridescence is a feature of some agates, in which concentric structures are so regular and closely spaced that they act as diffraction gratings. Iris agates display iridescence in transmitted light, and some do so in reflected light. Thin film interference is responsible for the play of colour of fire agates. At some stage, a bumpy growth surface of these stones was coated with a thin layer of a brown mineral, the thickness of which was just right to cause iridescence. When growth of the agate continued, the layer was enclosed and protected inside the stone.

Agates photographed by Pat Daly.


Any colour may be seen in agates but the commonest are grey to blue and brown to pink and red. When a cavity is only partly filled by agate, well-shaped crystals may grow in the remaining space. These may be amethyst or rock crystal, or of some other mineral, such as calcite. A nodule of this kind is called a geode.

The unqualified term chalcedony is usually taken to refer to material of a uniform colour. Carnelian (cornelian), for example, is a brown to orange stone which often shows agate structures. A darker, browner type is called sard and there is no clear boundary between the two varieties. Straight-banded white and brown to orange stones are called sardonyx. Like onyx, this is suitable for cameos and intaglios. 

Varieties of agate from the Gem-A Archives.


Types of Chalcedony: Chrysoprase

There are several green chalcedonies. Chrysoprase is a bright green variety coloured by nickel. Chrome chalcedony, also called mtorolite and by other names, is coloured by chromium. Prase, a poorly defined material described as dull leek green or light green, has been said to be quartzite (a rock), chalcedony and single crystal quartz. It is an excellent example of a term which has been so loosely applied that it is hard to use with any confidence. 

Dyed agate (left) and chrysophrase, photographed by Pat Daly. 


Dyed green agate is often seen in the jewellery trade. It may be distinguished from chrysoprase and chrome chalcedony because these rarely display agate structures, which are common in dyed agate. The Chelsea colour filter and the spectroscope can also be used to separate these stones.

What is Jasper?

Jasper is impure chalcedony. Colours are mostly red, yellow, green and white, often in patches and bands. Inclusions of iron oxides and clay minerals reduce translucency as well as adding colour, so jaspers are usually opaque, while most other chalcedony is translucent. Impurities cause the colours of some chalcedonies as well, so the dividing line between these materials is not clear cut. Jaspers do not usually display large scale agate structures, but small agate patches are common. 

Bloodstone from the Gem-A Archives, photographed by Henry Mesa.


Bloodstone, a dark green stone speckled with red, is the kind of jasper with which most people are familiar. It is widely used for jewellery, utensils and small carvings. Ribbon jasper is a rarer variety with parallel bands of green and red.

Advice for Gemmologists 

Confident familiarity with polycrystalline quartz gems is best achieved by looking at as many examples of these stones as possible, in gem dealers’ stocks, jewellery displays and museums. Photographs in books can be very useful and personal collections can be made, since most of these stones are relatively inexpensive. 

Aventurine beads from the Gem-A Archives.


It is well to keep in mind that most variety names were given by traders to communicate with their customers. Whilst there is a degree of consistency to the names, especially of the best-known varieties, adherence to rigorous definitions should not be expected, and much confusion may be avoided by accepting that boundaries between varieties may be uncertain. 

This does not, of course, excuse the practice of using quartz variety names for unrelated materials. The term jasper, for example, is misused for some limestones and volcanic rocks which are not quartz gems at all.


Main image: Quartz (aventurine) photographed by Pat Daly.


Gem-A Spotlights Demantoid, Spinel and Corundum in Latest Edition of The Journal of Gemmology


Respected scientific resource for Gem-A Members is now available in print and online to facilitate continued professional development for gemmology community.


Press Release: January 9, 2023

Gem-A, London, United Kingdom

The Gemmological Association of Great Britain (Gem-A) has launched the latest issue of its academic publication, The Journal of Gemmology (Volume 38, No. 4). 

Released in December 2022, this latest issue contains news and research from the international gemmological community, including a cover article that provides a mineralogical and gemmological overview of demantoid from Kerman Province, south-east Iran. The authors of the article, Drs Vahid Ahadnejad, Michael S. Krzemnicki, and Ann Marie Hirt, suggest that due to relatively high concentrations of the trace elements Cr, Ge, Ni and Co in Iranian demantoid, it can be separated from demantoid of other localities based on available chemical data. 

Additional feature articles include a focus on the spectroscopic characteristics and causes of colour modification in heat-treated pink-to-red spinel from Luc Yen, Vietnam; a study of the ‘Star of David’ pattern and presence of macrosteps on ruby and sapphire crystals from Aappaluttoq, Greenland; and an analysis of a new blue gem material known as fluorcarletonite. The latter article describes aggregates of this rare phyllosilicate mineral that occurs together with charoite in the Malyy Murun massif in Siberia, Russia. 

Editor-in-chief Brendan Laurs FGA says: “Continued professional development and learning is of paramount importance for gemmologists, whether they are starting their careers or have many decades of experience. This latest edition of The Journal of Gemmology includes feature articles that span detailed chemical analysis of gems, treatment methods, unusual crystalline structures, and the characterisation of new materials that can be creatively incorporated into jewellery and ornamental items. I hope Gem-A Members draw inspiration from this edition as we step into 2023.”

In addition to feature articles, each issue of The Journal of Gemmology contains a Gem Notes section, providing brief reports on diverse items of gemmological interest from around the world. In this issue, readers will find an update on emerald mining at Campos Verdes in Goiás, Brazil; take a closer look at interesting inclusions in purple spinel from Tanzania, and learn about the appearance of black sapphire melee as a black diamond imitation, among other reports. 

The What’s New column highlights recent publications and online resources useful for gemmologists, while the Learning Opportunities section contains detailed listings of upcoming conferences and educational events to further the professional development of its readers. Finally, the New Media section reviews new books and the Literature of Interest section lists recently published articles from sources worldwide that gemmologists can use to develop their knowledge further.

A leader in its field, The Journal publishes original research articles on all aspects of gemmology, including natural stones and their treatments, synthetics, and simulated gem materials. It is published quarterly in collaboration with the Swiss Gemmological Institute SSEF and with support from American Gemological Laboratories (AGL). 

Gem-A CEO Alan Hart FGA adds: “The Journal of Gemmology is testament to the remarkable research and discovery that’s taking place across the world. This quarterly publication is a much-anticipated resource for our Members, and we will continue to focus on publishing interesting and high-quality research as we move through 2023.”

Issues published in the past two years are accessible to Gem-A members both nationally and internationally. Previous issues are freely available to anyone via the Journal’s online archive. A cumulative index covering all issues from 1947 to 2022, as well as bibliographies of Journal articles covering specific subjects, can also be freely downloaded from the Gem-A website

Start receiving The Journal of Gemmology today.

Anyone can become an Associate Member of Gem-A and receive printed and online access to both Gem-A publications: The Journal of Gemmology and Gems&Jewellery. Membership costs just £135 per year and comes with a host of benefits. Visit the Gem-A website to find out more.

- ENDS -

For further information, please contact

Nysa Pradhan
+44 0207 404 3334


Notes to editors:

About Gem-A 

The Gemmological Association of Great Britain, or Gem-A, is the world’s longest established provider of gem and jewellery education. Our Gemmology Diploma evolved from the first gem course proposed for the UK jewellery trade in 1908, and our prestigious Gemmology and Diamonds Diplomas — taught in seven different languages and 26 countries around the world — are recognised globally.

Gem-A forms an international community of gem professionals and enthusiasts. We serve the interests of the gem and jewellery industries through high standards of education in our courses and our support for global gemmological research. We also provide various membership opportunities, offer high-quality gemmological instruments, and host a number of educational events throughout the year, as well as two internationally distributed publications, The Journal of Gemmology and Gems&Jewellery.

About The Journal of Gemmology

The Journal of Gemmology has been the official scientific journal of the Gemmological Association of Great Britain since it was incorporated in 1947. It is published quarterly in print and electronic formats. All Individual and Gold Corporate Members of Gem-A receive The Journal as part of their membership package. Institutional subscriptions are available to laboratories, libraries, museums and similar organisations. 

To contact the Editor-in-Chief, please direct correspondence to Brendan M. Laurs FGA at


Inside Look: What You’ll Learn on the Gem-A Gemmology Foundation Course 


If you’re considering our world-leading Gemmology Foundation course or want to find out more about the course’s contents, this is the article for you. Here, Gem-A Tutor Pat Daly explains more about this professional qualification, what resources you’ll receive, what the course contains, and how it can enhance your career. 


When students embark on the Gem-A Foundation course in gemmology, we jump into action to ensure they receive the right equipment to kickstart their studies. Each person receives the gemmology notes, a set of instruments in a folding case and a starter collection of gemstones and artificial materials on which to practice the skills they learn and develop throughout the year. 

The course introduces the concept of gemstones and explains why this group of materials has been of special interest to people worldwide and throughout time. Students will go on to consider gemstone properties and discover how they are related to the desirable features of the stones. These include their resistance to damage and the visual features which contribute to their beauty.

What Will I Learn on the Gem-A Gemmology Foundation Course? 

Gemstones are valuable items which are important in the global economy. They may be modified by treatment to make them more durable and attractive, and they may be simulated by factory-made products. An important part of gemmological work involves the separation of natural stones, their treated counterparts and artificial materials. These aspects of gemmology are changing in step with technological progress, and gemmologists must keep up with developments. The Gem-A Gemmology Foundation course enables students to practise some of the skills that are needed to progress in this fascinating subject.

The course begins with the definition of a gemstone, indicates the features which may be seen with a magnifying glass (loupe) on or in the stones on the syllabus and points out those which are used to identify them or, alternatively, to suggest the best way of doing so by using gem testing instruments. In many cases, gem identification may be carried out by using the tools supplied and the methods outlined in the Foundation course. The advent of new methods of treatment and of manufacturing synthetic stones means that, for many materials, the final step in identification depends on what may be seen with the loupe and microscope. 

Other instruments are introduced as they become relevant. Those to do with colour, for example, are described when that feature of gemstones is considered.

What Gemstones Are Covered in the Foundation Course? 

Twenty gem species are described in detail in the course, together with their appearances, properties and how they can be identified. The Foundation course concentrates attention on those stones which have the greatest commercial importance. They include diamonds, emeralds, rubies, sapphires, garnets and quartz gems, amongst others. Other species are described in the Gem-A Gemmology Diploma course. 

Many of these are interesting or beautiful, some are seen commonly in the jewellery trade, and they are by no means unimportant. Nonetheless, the Foundation course aims to inform students about those stones which are the most significant in terms of their abundance, beauty and value, and which supply most of those which are seen in jewellery. 

The Science of Gemmology 

Gemmology is a science-based subject. It operates by the evaluation of evidence gained from all the instruments that are available to the practitioner, from the simplest ones which are used at the start of the course to advanced types which may never be seen by most gemmologists. The same principles underlie the use of all equipment, however, and it is necessary for us to learn those on which our gemmological tools are based. 

In particular, knowledge of the optical properties of stones and instruments, though they may be troublesome to learn for those who are not already familiar with them, is essential if advanced testing techniques using standard equipment are employed during commercial gem testing. Some additional scientific knowledge is useful in understanding instruments which, currently, are located mostly in laboratories. Although most of us will not use them, it is important that, when we tell clients or employers that we cannot give a definite answer to a gemmological question and that a stone should be sent to a laboratory, we speak from a position of knowledge and experience and not one of ignorance. 

Creating the Foundations of Your Future Career

Much of this learning is for the future. The Foundation course, as its name suggests, serves as an introduction to gemmology, but it briefly covers the topics which are considered more fully in the Diploma course and which gemmologists can develop further when they have successfully completed these two.

Gemmology is a complex subject, like so many others nowadays, because so much scientific and technological progress has been made during the last few decades. The basic features of the subject remain the same, however. The gemmologist needs to accumulate knowledge of gemstones and instruments and acquire experience in handling stones effectively. 

Caring for Gemstones 

Gemstones are valuable, sometimes very valuable, and must be cleaned and tested with the greatest care. An accident which causes damage to a stone may involve the handler in an expensive repair or replacement. Gem testing begins with the eye and proceeds through magnification to the skilled use of instruments. Wrong conclusions, caused by careless testing or insufficient knowledge, may also be expensive, so students should adopt the sensible procedures described in the course to avoid damaging mistakes.


Despite these concerns, those who wish to study gemmology should do so with persistence and the determination not to be overawed by apparent difficulties, which may prove to be much less daunting when approached after the basics of the subject have been learned.

It is also important, from time to time, to remind ourselves of the reasons why we became interested in gemmology and of the pleasure which we continue to enjoy in its pursuit. These will inspire us to follow our interest in the harder parts of the subject and reassure us that our efforts will be rewarded.


If you have further questions about the Gem-A Gemmology Foundation course, speak to a member of our team via

If you would like to sample the subject of gemmology before embarking on this course, consider our online-only GemIntro platform. This is a quick, accessible and affordable way to learn the basics of gemmology. 

All images from the Gem-A archives.