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Understanding Common Corundum Treatments

Rubies and sapphires are often treated to improve their colour and appearance, but what are these treatment methods, and how can gemmologists pinpoint them? Here, Gem-A Tutor Pat Daly FGA DGA talks us through some of the most common methods used for corundum. 

Work is carried out on nearly all gemstones before they are offered to consumers. At the least, crystals and crystal groups are trimmed and shaped, and most stones are cut and polished to make them suitable for use in jewellery. Stones in these conditions are regarded as untreated, but a high proportion are modified in other ways to make them more attractive. 

Corundum, which includes the gem varieties ruby and sapphire, is very often treated, and this is likely to have been the case for more than 1,000 years. Early treatments included dyeing and heating using charcoal fires and blowpipes. Since the 1970s, increasingly sophisticated treatment methods have been used to bring many otherwise unsaleable stones to the gem market, improving the supply of gem quality stones and making attractive, affordable stones available in commercial quantities. 

Rough corundum crystals from the Gem-A Archives

Heating can now be carried out at higher temperatures than is possible with traditional methods, chemical elements may be driven into stones, and their clarity may be improved. Those who handle gemstones must know how rubies and sapphires can be treated and the clues that may inform us that they have been carried out.

Corundum Treatments: Dye 

Dyes are used to add colour to low-quality corundum. Pigments suspended in oil or water are deposited in fractures and pits. Treatment is recognised by the concentration of colour in fractures and the staining of solvent-soaked swabs wiped across surfaces. In jewellery incorporating closed-back settings, coloured foil placed behind a stone usually shows some crumpling, which can be seen with a loupe. 

Corundum Treatments: Heat

Heat treatment can improve colour and transparency at temperatures ranging from about 800 to 1900°C. Temperatures less than about 1000 to 1100° tend to lighten blue colours. This may be desirable to remove a modifying colour from rubies or to lighten the colours of sapphires which are too dark. There may be few clues to this relatively low-temperature treatment, but iron staining in fractures may change colour from yellowish-brown to reddish-brown. 

Ruby with crystal and feather inclusions, photographed by Pat Daly.

Higher temperature treatment affects the appearance of crystal inclusions and feathers and may be easier to recognise. Colours are modified by the intensification or reduction of blue and yellow. Those colours may become more vivid, or they may be used, for example, to turn a pink stone into the more favoured pinkish-orange padparadscha colour.

Heat treatment modifies the condition of the elements iron and titanium, which can be present as impurities in corundum. Both may combine with oxygen to crystallize as oriented needle-like inclusions. When there are enough of them, they may cause a star effect, but they can also occur as patches which reduce the clarity of stones. Heating can dissolve them so that the elements are incorporated in the corundum, and clarity is improved. These two metals interact to produce the blue colour of corundum, so this feature may be improved simultaneously. 

A synthetic star sapphire, photographed by Pat Daly.

Evidence of the former presence of needles may remain as lines of tiny dot-like inclusions or of small colour patches along the directions of the inclusions. These features have been called dotted silk and cross-hatched colour zoning, respectively. Straight colour zoning, seen in most blue sapphires, becomes foggy looking, the sharp boundaries between blue and white zones becoming blurred as colouring elements diffuse short distances in response to the elevated temperatures. 

Angular colour zoning in sapphire, photographed by Pat Daly. 

Fractures often develop around other crystals, producing features called heat discs, and when temperatures reach about 1500°, previously transparent crystals may become whitish and translucent, taking on a “frosted” appearance. Feathers, thought to represent fractures partly healed during the growth of gemstones, undergo subtle changes, but experience is needed to interpret them correctly.

Heat treated sapphire with heat disc features, photographed by Pat Daly.

Corundum Treatments: Diffusion

Colouring elements can be diffused into white sapphires, which do not improve after heat treatment. Pre-cut stones are packed in aluminium oxide containing titanium and iron and heated at up to 1850°C for some days. Colouring elements penetrate a short distance beneath the surface. Therefore, the stones must be repolished, removing some colour from the facets. When seen against diffused lighting, ideally when immersed and viewed from the pavilion side, it is usually noticed that some facets are paler than others and that colour is stronger along facet edges. Colour diffusion of chromium produces pink to red, and nickel is used for yellow stones. 

Titanium may be diffused into cabochon cut stones to improve a star. Subsequent heating causes needle-like crystals to grow in three directions, and these reflect light to produce the optical effect. In most untreated star stones, needle-like crystals are seen easily with a loupe, or the straight zones in which they are concentrated are prominent features of the stone. Suspicion should be aroused when neither of these features is seen. Suitable heat treatment may improve a star without surface diffusion if there is enough titanium dissolved in a stone.

Needle like rutile inclusions in sapphire from the Gem-A Archives.

The element beryllium may be diffused further into corundum and affect the whole stone, producing a wide range of colours. The features noted above for diffusion-treated stones are not seen, and these stones may be hard to recognize, though evidence of high-temperature treatment is usually visible.

Corundum Treatments: Clarity

Heat-treating stones in contact with a flux, such as borax, may partly repair fractures. This treatment is routine for stones from Mong Hsu in Myanmar, and those from other localities may be improved by it. However, the resulting feathers are similar to natural ones; some experience is needed to separate them.

Glass-filled ruby with iridescence in direct light, photographed by Pat Daly.

Fractures in poor-quality corundum may be hidden by filling them with oil or glass. Glass-filled rubies from Mozambique are recognised by iridescence, seen in fractures at low viewing angles, and by a change of lustre when wider cracks intersect the surface. Fracture-filled stones are much less valuable than good-quality rubies, and standard workshop techniques may damage them, so it is important to identify them.


Glass-filled ruby from the Gem-A Archives.

Pale, heavily fractured corundum may be filled with glass, at least some of which is coloured by cobalt. When seen in diffused transmitted light, colour is concentrated in fractures, the stones look red through a Chelsea colour filter, and they may display a cobalt spectrum. They also show the same features as glass-filled rubies.

Corundum Treatments: Irradiation

Irradiation may produce an unstable yellow colour in sapphires. This treatment cannot be detected except by a fade test; some stones from a parcel are exposed to sunlight for some time. Naturally coloured sapphires may fade in intense artificial lighting, but the colour will usually intensify again if the stones are held in sunlight or ultraviolet.

The treatment of corundum to modify colour and/or clarity is widely practised and serves to bring stones to the jewellery trade, which could not be supplied just by using untreated stones. The improvement of gemstones by treatments is acceptable, so long as buyers are told that they have been carried out and of any issues connected with durability, such as colour fading or inability to withstand heat in workshops. 

Those in the gem and jewellery trades should keep abreast of developments in this area of gemmology, pay attention to any ways that treated stones may be identified, and consider sending doubtful stones to laboratories to confirm their status. 

Main image: A sapphire with abraded facet edges, photographed by Pat Daly. 


Beginner’s Guide: Interesting Inclusions in Diamonds


Inclusions in diamonds are typically considered a negative trait, especially where their commercial value in the jewellery trade is concerned. But, as Gem-A Tutor Pat Daly explains here, inclusions in diamonds can be a fascinating area of study for gemmologist. Let’s consider some of the ‘disruptions’ to a diamond’s carbon lattice and uncover more about what makes inclusions so interesting. 

When measured by value, diamonds are the most important gemstones in the jewellery trade. Their quality is assessed in terms of the 4Cs; clarity, colour, cut and carat weight. The ideal diamond to be set in jewellery has no internal feature that an expert can see at ten times magnification. Accordingly, feelings about inclusions in diamonds are largely negative, especially if they can be seen easily with a loupe. 

However, those who are interested in diamonds may find inclusions beautiful and intriguing in themselves and appreciate the information they give about their growth and the otherwise inaccessible regions of Earth from which they come. Insights may be gained into the Earth’s mantle, its composition, and the connection between it and the movement of continents and oceans by plate tectonics.

Diamonds in conglomerate matrix photographed by Henry Mesa.

Some inclusions reveal the treatment of diamonds to change their colours or to modify their inclusions. Others supply evidence which can be used to separate natural from synthetic stones. 

Common Inclusions: Diamond Cleavages 

The commonest inclusions are cleavages, known as feathers in the diamond trade, and the black mineral films which sometimes coat them. Diamond is a tough material, but when it breaks, it does so more readily along flat surfaces, in four directions which are parallel with octahedral crystal faces. They are not desirable inclusions, but they are useful to gemmologists. Convincing diamond simulants do not possess this property, so it proves the identity of some diamonds. Many cleavages are thought to occur during the rapid and violent ascent of the volcanic rocks, which bring diamonds from the depths to the Earth’s surface.

An included diamond from the Gem-A Archives, photographed by Henry Mesa.

Diamond Colour Zoning 

A common feature of brown and pink diamonds is straight colour zoning parallel with the crystal faces of the growing stones. A feature called graining, which might be described as a kind of zoning recognised by optical features other than colour, is probably related to it. When it is subjected to stress within the Earth, diamond, in common with other minerals, may change its shape without breaking. However, it does so in certain definite directions in the crystal, and traces of this distortion may be evident as colour zones or graining. 

Irradiated Diamonds 

Some stones of poor colour that have been irradiated or may have been heat-treated to change them to attractive colours also show colour zoning. One of the most striking types has a shape resembling an open umbrella around the culet. This is seen in some stones which were irradiated by electrons in a machine called a cyclotron. Other types of colour zoning may be seen in stones irradiated in the past, but modern treatments tend not to leave such evidence. 

Crystals of Minerals within Diamond 

Tiny, indeterminate inclusions may be grouped into geometric shapes. For example, inclusions which resemble minute sugar cubes are seen in some diamonds. 

Dark inclusions in a diamond photographed by Henry Mesa.

Crystals of minerals which grew before, or at the same time as the host diamond, may be preserved within polished stones. Recognizable crystals are rare, occurring in about 1% of diamonds, but they tend to be eye-catching, and they can be very attractive. Despite their infrequency, it is easy to compile a list of 30 or more minerals by looking through recent articles in gemmological journals and standard works on gemstone inclusions. 

Diamond crystals, for example, may be swallowed up by larger stones and may be recognised by their octahedral forms. Where one of them intersects the surface of a polished stone, a raised feature called a naat might result from differential hardness effects when the crystal structures of the two diamonds are not in alignment.

Diamond Growth and Inclusions 

Most diamonds grow at about 100 to 250 Km below the surface of continents in two main rock types. It has been calculated that about two-thirds form in peridotite, a rock composed largely of olivine (of which the gem variety is peridot) and other minerals such as pyrope garnet and diopside. Transparent, white olivines are among the commonest crystal inclusions in diamonds. Chromium is a noteworthy minor chemical constituent of peridotite. It causes the bright red colour of pyrope and the bright green of diopside inclusions, indicating that diamonds formed in this rock type.

Diamond in conglomerate matrix, photographed by Henry Mesa.

The other type is eclogite, the high-temperature and pressure metamorphic equivalent of common ocean floor rocks. It can be a visually striking rock composed of green pyroxene and red garnet. Pyroxenes belong to a group of common rock-forming minerals, some of which are familiar to gemmologists. The one in eclogite is intermediate between jadeite and diopside. The garnet is intermediate between pyrope, almandine and grossular. Chromium is less abundant in eclogite than peridotite, and the colours of these crystals incline towards orange in the garnets and greyish green in the pyroxene, in contrast to the bright colours of those materials in stones from peridotites. 

Blue crystals of kyanite and brownish-to-black rutile inclusions may also be found in diamonds from eclogite.

Rare Diamonds 

More rarely, diamonds that grew at greater depths, between 400 and 800 Km, are brought to the surface. They are of great interest because they may be large stones of good colour and clarity. Some of their mineral inclusions are not found in rocks from higher levels in the Earth or, if they are, they are not seen in diamonds which grew at shallower depths. One of the commonest consists of mixtures of iron and nickel compounds, leading researchers to suggest that diamonds grew from liquids incorporating these molten metals. 

Removing and Adding Inclusions in Diamonds

Dark inclusions may be bleached by using a laser to burn a hole into them or to heat and expand them so that a crack opens to the surface of the stone. In both cases, strong chemicals can then be used to bleach them. The channels along which they are introduced supply evidence of the treatment. Cracks may be disguised by injecting them with glass. Bubbles and iridescence at low viewing angles reveal these fracture fillings. 

Fracture-filled treated diamond with a characteristic ‘colour flash’ from the Gem-A Archives.

Synthetic diamonds sometimes contain useful inclusions. For example, those grown at high temperatures and pressures in a nickel-iron flux may enclose some of it. It may be recognisable and, if large enough, it can make the diamond magnetic; this property is rarely found in natural stones. 

A laser drilled and fracture filled treated diamond with colour flashes and entry holes, from the Gem-A Archives. 

Manufacturers may laser-engrave the girdles of their synthetic diamonds, and one has even induced inclusions to mark them. For example, De Beers has engraved a logo beneath the table facets of its laboratory-grown diamonds in its Lightbox range.

Inclusions in diamonds are unwelcome from a commercial point of view but may be extremely useful in separating natural, treated and synthetic stones. In addition, they may serve as distinguishing features for individual stones, supply evidence of the conditions in which they grew, and some are welcome features to the gemmologists who are privileged to see them.



Beginner’s Guide: Understanding the Process of Diamond Grading 


What does diamond grading actually mean, and why is it important? If you are new to the world of gemmology and want to get to grips with the study of diamonds and precious gems, this is a helpful place to start. Here, Gem-A Tutor Pat Daly shares beginner’s level insights into grading and assessing a diamond for its characteristics, known as the 4Cs. 

The relative values of diamonds have been important since they were first traded. Larger stones were more appealing than smaller ones, for example, but size has never been the only consideration. 

Diamonds are attractive because they can be of any colour, and their optical properties mean that well-cut stones return more of the light which falls on them to an observer than any other gemstone; this feature is called brilliance. In addition, some of the white light is split into spectral colours, and diamond is one of the best gemstones to display this property, known as fire.


A rough diamond crystal photographed by Henry Mesa. 

Over time, the factors that make one diamond preferable to another have been refined so that they may be considered under four headings, known as the 4Cs. They are Carat weight, Colour, Clarity and Cut. The value of a diamond depends on the combination of these factors.

Diamond Carat Weight 

The size of a diamond measured in carats, the standard unit of weight for gemstones, is important because, other factors being equal, larger stones are more valuable than smaller ones, not least because they are rarer. It has been calculated that the average size of a rough diamond from a mine is less than 0.10 carat, and this weight may be reduced by 50 to 60% by polishing. A faceted diamond weighing more than one carat is a rare stone.


Rough diamond crystal photographed by Pat Daly from the Gem-A Archives.

Diamond Colour  

A diamond may be any colour, but most stones vary between white and yellow or brown. This range is divided into 23 categories by GIA denoted (D to Z), of which the most favoured are those with the closest approach to pure white (D colour). Under controlled conditions, a stone is graded for colour by looking obliquely through the pavilion while it rests on a white surface. The process requires careful assessment of the stone by a practised observer. 

At the lower end of the scale, the colour becomes more evident, and graduates to definite, desirable colours called fancy colours. For the commonest colours, yellow and brown, there is some overlap between the D-Z range and the descriptions used for fancy stones. This happens because fancy-coloured stones are assessed by looking through the crown of the stone rather than the pavilion. This means that a diamond cutter may polish a stone in a way which accentuates the colour, shifting it, for example, from the white to yellowish range to fancy yellow. 


A round brilliant-cut diamond from the Gem-A Archives.

For rarer colours, such as pink, green and blue, any colour seen through the crown of the stone places it in the fancy colour range. The grading of fancy-coloured diamonds is a specialised task which cannot be safely undertaken by anyone who has not been trained to do it. 

Diamond Clarity 

 Clarity refers to the abundance of inclusions in diamonds. A diamond will look more attractive if there are no inclusions to distract the eye or disturb the passage of light through the stone. A grading system has been developed based on the number, size, colour, and position in the stone and any effects they may have on its optical properties. The most desirable stones have no internal features which an expert can see at ten times magnification in properly controlled conditions. 


Mineral inclusions in a faceted diamond from the Gem-A Archives.

The position of inclusions is significant. A crack or a crystal, for example, located near the girdle of a stone, is not considered to be so objectionable as a feature of the same size and colour in the middle of the table facet. 

Diamond Cut  

The fourth C – cut - is equally important. Most diamonds do not have a definite, desirable colour. Instead, they are valued for their brilliance, sparkle and fire. Brilliance is the return of light which falls on a stone. Sparkle denotes the movement of reflections from facet to facet as the stone is turned. Fire is the splitting of some of the white light in the stone into spectral colours. These effects can be optimised by careful design so that the facets of a cut stone have the best relative sizes, shapes, positions and angles. 


A closer look at diamond facets photographed by Henry Mesa.

The quality of polish of the facets is important because they act as windows and mirrors on the stone, allowing light to enter and leave it as efficiently as possible. Poor polish will scatter some of the light and detract from the overall appearance of a stone. All these factors are judged when deciding the cut grade of a diamond. Informal terms such as light performance have been used to indicate how a stone affects light, but they are not used in diamond grading because they are imprecise.

Finding Compromises Between the 4Cs 

When preparing diamonds for the jewellery trade, compromises must be made between the 4Cs, so that the best value is obtained from each diamond. Size may be reduced to cut out a large inclusion. As a result, the value of the finished stone is increased even though it is not as large as it could be. The faceting style is affected by the shape of a rough diamond. 


A diamond under the microscope photographed by Henry Mesa.

The round brilliant cut is generally reckoned to be the best style for a diamond, but the outline shape of a crystal is square, at best, so much material is lost during fashioning. A stone of less regular shape may be cut to an oblong or pear shape to minimise weight loss in cutting. This reduction increases the value of the finished stone even though the shape is not ideal. 

Many examples could be given where one factor is promoted at the expense of another, by diamond polishers and by retail customers who might be prepared, to some extent, to sacrifice clarity or cut quality to obtain a larger stone. 

What is Diamond Grading?

Diamond grading involves the skilled appraisal and careful consideration of all aspects of the 4Cs. Experienced professional graders, using well-designed and costly equipment in ideal lighting conditions, with access to databases and the benefit of the opinions of fellow professionals, may work without distraction to arrive at their opinions. As a result, certificates issued by their laboratories are widely used as independent opinions that traders and their customers can trust.


The girdle of a faceted diamond from the Gem-A Archives.

Independent traders who do not have these advantages may still need to make an independent assessment of the quality of a diamond when buying and selling or when verifying that a stone corresponds to the description given in a certificate to guard against substitution. This may be done by attending courses designed to equip them with the necessary skills and accepting that they will not be able to compete with professional graders without further experience.

We offer a five-day laboratory class at Gem-A London dedicated to diamond grading. This is an ideal way to enhance your knowledge and learn in more detail. Elsewhere, the Gem-A Diamond Diploma is offered on-site, via our Accredited Teaching Centres (ATCs) and through Online Distance Learning (ODL). This more in-depth and professional qualification also incorporates practical work in handling diamonds. Find out more by emailing us at



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.