ENHANCEMENT

The colour and appearance of natural and synthetic gemstones can be enhanced by methods such as oiling, dying, staining, foiling, heat-treating, iridating, drilling and filling. Heavily included gemstones, such as emerald, are oiled to fill cracks and give the stone a clearer appearance. Oiling has been carried out for more than 2000 years, and is an acceptable part of the business. Filling the cracks with oil, wax and stain may be temporary, but more long lasting resins have recently be introduced.

Oils and stains may leak and can be identified by wiping with a soft cotton cloth. Turquoise, Lapis Lazuli, Jade and Agates may be given a coating of wax. Agates are often stained to bright colours that have no natural counterpart and are therefore easily recognized as fakes, though some find them attractive. Placing a piece of foil behind a mounted gemstone to increase its fire and sparkle was a method used by Victorians, particularly for costume jewellery. Probably the oldest method of enhancing the appearance of a gemstone is by heating. In India, Carnelian has been heat-treated for more than 4000 years. Heating may be carried out in oxidizing conditions or in reducing conditions. Heat treating and irradiating gemstones can be used to change or enhance colour. For example, colourless and pale brown Topaz can be heat-treated to give blue Topaz. In addition lasers can be used to drill even the hardest diamonds to remove inclusions and the drill hole may then be filled.

INCLUSIONS IN SYNTHETIC GEMSTONES

A perfectly clear stone, without flaws or inclusions (internal features of gemstones, such as solids, liquids or gases, that have become enclosed within the gemstone during or after formation) is usually a warning sign to a gemologist that the stone may be synthetic. However some synthetic gems may be identified by the type of inclusions, formed as a result of the method of manufacture. Flame-fusion gem-stones may include specks of the powdered ingredients that have failed to melt sufficiently, and have curved growth-lines, rather than the straight lines that develop in natural specimens.

Synthetic-flux emeralds may have inclusions of the minerals phenakite or platinum, twisted veils and feathers (characteristic patterns of inclusions), and two-phase inclu­sions (a liquid and a solid). Synthetic hydrothermal emeralds usually have only a few inclusions, possibly of phenakite and occasionally very fine two-phase inclusions.

natural inclusions

Natural inclusions may be specific to one gem or even to a particular country or mine. Emeralds, for example, tend to be so heavily included that the view down a microscope (X40 magnifica­tion) is sometimes referred to as jardin (French, 'garden'). Natural emeralds may have inclusions of the minerals mica, tremolite, actinolite, pyrite, or calcite. Colombian emeralds may contain characteristic three-phase inclusions (solid, liquid, and gas), with a cubic salt crystal and a gas bub­ble within a fluid-filled cavity. Other gemstones with characteristic inclusions include peridot, which has inclusions that resemble 'lily pads' (liq­uid droplets around a crystal of chromite) and the 'treacly' appearance of hessonite garnet.

SYNTHETICS

Gemstones can also be made in the laboratory. Where they have the same chemical composition as their natural counterpart, they are referred to as synthetic. Synthetic gemstones therefore have virtually the same physical and optical properties of the natural gemstone.

The ruby was the first gemstone to be made artificially. In 1837 a French Chemist Marc.A.Gaudin tried to produce some synthetic rubies but eventually gave up, admitting defeat in the published notes of his final ruby experiments as they were not of any value as gems because they became opaque as they cooled.

In 1877, the French Chemist Edmond Fremy (1814-94) and a student assistant developed a method what is now known as Flame Fusion method. They heated 44.1-66.15 lbs (20-30 kg) of a solution of Aluminium Oxide dissolved in Lead Oxide in a porcelain vat for 20 days. As the solvent evaporated and che,mical reactions took place among the solution, the vessel and furnace gases, a large number of very small Ruby crystals formed on the basin's wall but the crystals could not be used in jewellery as they were very small.

Later in 1891, a French Scientist Auguste Verneuil (1856-1913) developed a somewhat different process that eventually became successful. He did not publish a description of his technique until 1902. His assistant exhibited the synthetic rubies in 1900 at the paris World's Fair, where they were quite popular. By the time Verneuil died at the age of 57 in 1913, the process he had invented was being used to manufacture 10 million carats of rubies annually.

NON-FACETED AND FACETED GEMS

Gems that are heavily included or flawed, coloured or patterned, opaque or translucent are generally fashioned as beads and carvings or cut as a cabochon. A cabochon consists of a domed polished top with a flat, unpolished back and has an oval or round outline. It is the simplest and oldest style of cut. It is also used to show the optical effects of iridescence, sheen, cats'-eyes (or chatoyancy), and star stones (or asterism).

Most transparent gemstones are faceted. The gems are cut and polished such that they have a number of flat polished surfaces (facets). The number of facets and the angles between them are worked out mathematically, so that the facets act as mirrors that allow the maximum amount of light to enter the stone and also be reflected

back out of the front of the stone to the viewer. A well-cut gemstone will be cut to show the body colour, brilliance, fire, and sparkle to its best effect. However, a compromise usually has to be found to reach a good balance between these four attributes, which will give the final appearance - referred to as the 'make' of the stone. It may also be necessary to retain as much weight as possi­ble, as the weight of the gem directly influences its value.

FASHIONING AND CUTTING

Gem materials have been fashioned and polished for millions of years- and if they have been left in their rough state, it would be because they look just fine as they are, and their natural beauty and desirability is worth retaining. For others, there may be a belief that the strength, soul or energy of the gemstone may be released or in someway affected detrimentally by intervention.

The vast majority of gem materials, however, can be improved by the creative and practical skills of the artist and scientist. As tech­niques and skills have developed, the range of possibilities for the sculptor, carver, or the lapidary and others has grown, but the aim has remained the same - to work with natural materials to bring out their best qualities.

The transformation from the rough gem (such as a pebble, crystal, or crystal fragment) to the finished product (a valuable, sparkling gemstone) depends upon the expertise of the lapidary. The style of cutting or fashioning chosen will depend primarily on the optical and physical properties of the gem and the shape and condition of the rough stone.

STABILITY

In addition to physical stress due to an impact, light, heat, or some other event, chemicals may alter the structural and surface condi­tions, particularly of the organic gemstones such as pearl and amber. The chemicals in some perfumes, for example, will act to wear away the outer surface covering of pearls, removing their glimmering lustre. For a gem to be stable, it must be able to resist both chemical and physical alteration.

TOUGHNESS

Toughness in a gemstone is a measure of how well it can resist a fracture developing, i.e. how far a crack will propagate. Gemstones that are polycrystalline (made of more than one crystal/grain), such as nephrite ,jadeite, and AGATE (polycrystalline quartz variety), are gen­erally tough and difficult to break. The interlocking crystal grains are oriented in different directions, thereby slowing or stopping the propagation of the fracture.

zircon is brittle, it is easily chipped and the corners break off as it cleaves easily. A great deal of care and attention is therefore required at all stages of fashioning by the lapidary, and later in terms of storage by the retailer and ultimately the owner.

FRACTURE

Fracture is a random, non-directional break that can be caused by impact, stress, pressure, or a rapid change in temperature. Fracture is not related to planes of weakness in the crystal structure and can therefore occur in both crystalline and amorphous (non-crystalline) materials.

The fracture surface may be described for example as uneven, irregular, conchoidal (shell-like), or hackly (uneven and jagged).

CLEAVAGE

The internal arrangement of atoms within a crystalline material (the crystal structure) has a regular three-dimensional pattern. The atoms may be bonded in a number of differ­ent ways, with bonds of different strengths. As a result, the crystal may have one or more planes or directions of relative weakness along which it will break more easily. These are called 'directions of cleavage'. When a crystalline material breaks along a cleavage plane, it will leave a flat, or nearly flat, cleavage surface. Cleavage can only occur in crystalline materials and may be defined as perfect, good, fair, or poor.

diamond is the hardest gemstone, but with a well-aimed hit, it is possible to break a diamond in two with a clean break. The clean break occurs along one of its three perfect cleavage planes.

This ability to cleave is taken into account during the fashioning of a diamond, and may be the first step in the process of cutting. The ease with which a gemstone cleaves and the number of cleavage directions it possesses is also important with regard to its durability and its identification.

STREAK

When a softer mineral is rubbed or scratched across a harder surface, a fine layer of the softer mineral may be grinded and deposited as a fine dust or a coloured mark referred to as a 'streak'. For example, the writing left on paper when a graphite pencil is used.

Some minerals leave a characteristic streak, for example the dark blood-red streak of haematite and the grey streak of galena. Testing for streak is destructive and is not generally undertaken by gemologists, particularly as it is of little use with small, cut stones. However, it may be useful with larger uncut specimens, rock fragments and decora­tive pieces, as well as to check for dying or colour impregnation. For example, malachite has a green streak, but another rock type dyed green to imitate malachite may have a white streak and only a thin green dyed layer.

PHYSICAL PROPERTIES OF GEMSTONES

The manner in which the atoms of a gemstone are arranged and the strength of the bonds between them directly affects the physical properties of gem stones, their durability (hardness, toughness, and stability), the way they break or cleave and their relative density (specific gravity).

The ability of a gemstone to withstand general day-to-day wear and tear sufficiently to be mounted in a piece of jewellery and to keep its polish without becoming unduly scratched, cracked or worn is important and will affect its value. Generally speaking, the harder the gemstone the better it can take and retain a good polish.

Hardness

In gemmology, hardness is a measure of how easily a surface can withstand abrasion caused by wear and tear including scratching. If jewellery items containing a range of different gemstones are stored together, the harder gems such as ruby and sapphire will scratch the less hard pieces, for example emerald, amethyst, or opal.

Because hardness is related to the crystal structure and the strength and direction of the atomic bonds, the gemstone may be scratched more easily in one direction than another. This differential hardness is particularly important for the lapidary when assessing how best to cut and polish a gemstone and in choosing a suitable mount or setting for the piece.

MOHS' SCALE OF HARDNESS

The relative degree of 'scratch ability' or susceptibility to scratching can be assessed using the Mohs' scale of hardness. In 1822 Friedrich Mohs, a German mineralogist took ten minerals that he was able to obtain easily and tested them against each other. Mohs arranged the minerals in order, from the softest (talc) that could be scratched by all the others, to the hardest which could not be scratched (diamond). He then assigned them numbers from the softest (1 = talc) to the hardest (10 = diamond).

The test is a comparative one, and the scale is not linear and does not increase by equal increments, for example the difference in hard­ness between 1 (talc) and 9 (the corundum group, which includes ruby and sapphire) is less than that between 9 and 10 (diamond).

Other hardness scales include: a) the Brinell Scale, which measures how much of a dent can be made when a steel ball is pushed into the surface (this cannot be used on particularly brittle, thin or fragile pieces, as they would break);

b) the Knoop scale which uses a diamond to make a measurable dent; and

c) the Vickers scale which also uses a diamond to make a dent.

The Mohs' test for hardness is quicker, cheaper and easier than the other methods. An estimate of hardness can be given by looking at the general wear and tear of the crystal faces (on an uncut 'rough' specimen) or flat surfaces (facets) and facet edges on a cut and pol­ished gemstone. A set of hardness pencils (each with a point made of one of the ten mineral specimens) can be used to test hardness and is particularly useful on carved pieces, crystal fragments and pebbles.

COLOURED GEMSTONES

The colour of a gem largely depends on how it absorbs or reflects light energy. When white light strikes a gem, some of the spectral colours are absorbed in preference to others (preferential absorption). Those that are not absorbed pass through the gem or are reflected back, giving the gem its colour.

Some gemstones have a characteristic absorption spectrum that, with the use of a gemological instrument called a spectroscope, can be used to assist identification, or for example to distinguish between two gemstones with a similar appearance but different spectra, such as ruby and garnet. Through the spectroscope, the absorption spectrum of the gemstone looks like an incomplete rain­bow, with black lines or bands replacing some of the colour. The lines and bands signify the energies with wavelengths that corre­spond to those colours that have been absorbed; the remaining energies give the gem its colour.

Idiochromatic And Allochromatic Gemstones

Where the colour of a gemstone is caused by elements that are an essential part of the chemical composition, the gem is termed idiochromatic (from idio meaning same and chroma meaning colour), for example the green colour of peridot is due to iron (Fe), an essential part of its composition (FeS₂).

Most gemstones are allochromatic (from allo meaning other) and are coloured by small amounts of other elements or impurities called trace elements that are not an essential part of the chemical composition. The most common trace elements are the metals chromium, vanadium, iron, titanium, copper, and manganese. The bright green of emerald and demantoid garnet is caused by chromi­um, which also gives the bright red of ruby. sapphires are coloured blue by iron and titanium, or green, yellow or brown by iron. Man­ganese colours spessartine-garnet orange and rhodonite pink. Heat­ing, irradiation, and other means of altering, enhancing of destroying colour, are more likely to be effective on allochromatic gems than idiochromatic gems.

PARTI-COLOURS

During and after gemstone growth, changes in the surroundings may result in different trace elements being available for incorporation into the gemstone. These may result in a change in colour, colour banding, or patches of colour. When one part of a gemstone is a different colour to another part, it is called parti-coloured. Watermelon tourma­line, with its pink interior and green outer rim, is an excellent example of a parti-coloured gemstone, others may have more than two differ­ent colours.

Pleochroic (Dichroic And Trichroic) And The Dichroscope

A gemstone may also appear different colours or shades of colour when viewed from different directions (pleochroic). This is as a result of the way light travels through the crystal structure of the gemstone. When a gem shows two different colours or shades of colour, such as in some rubies, the gem is described as dichroic. Trichroic gem-stones such as iolite (cordierite) and tourmaline show three colours or shades of colour.

To see the different colours you have to view the gemstone from one direction, remember the colour, and then turn the stone and compare the image with the colour when viewed from the new direction. Slight changes in colour can be difficult to recognize. The dichroscope, a handheld gemological instrument with a polar-oid filter and similar in size to a hand lens (loupe), can be used to view two colours of a pleochroic gem at the same time enabling comparison to be made more easily.

REFLECTION EFFECTS IN GEMS AND STONES

Interference - iridescence and play of colour

Rainbow effects such as those seen in cleavage cracks and the irides­cence of labradorite, feldspar and haematite are due to interference as light is reflected by thin layers (films) within the gemstone. In moon­stone feldspar, the effect is known as schiller (sheen), adularescence or opalescence and the iridescent colours on the surface of the pearl is called the 'orient of pearl'. As light is reflected, there is interference of the wavelengths. Where they coincide, the colour corresponding to the wavelength may be enhanced, in other places waves may cancel each other out and that colour will no longer be seen.

In opal, interference of light occurs as it passes between the regu­larly arranged spheres that make up its structure. The size of the spheres and the distance between them affects the amount of disper­sion and resultant play of colour; as well as the direction from which the opal is viewed. Small spheres produce only the blues and violets as the opal is turned, while large regularly packed spheres show the full range of rainbow colours.

chatoyancy, asterism, and silk

Other internal reflection effects include chatoyancy, asterism and silk, which are caused by inclusions. Chatoyancy is the cat's-eye effect seen on some gemstones that are cut as a cabochon (polished as a rounded dome) and best seen under a bright light such as a spotlight or torchlight. Light is reflected off a parallel arrangement of elongated or acicular (needle-like) crystal inclusions of minerals such as rutile or tourmaline, fibres or long tube-like cavities. Examples of gemstones that may show a cat's-eye include quartz, beryl, ruby,

SAPPHIRE, and TOURMALINE.

Asterism forms star-stones where there are two or more sets of parallel inclusions instead of a single set. Stars may have four, six, twelve, or even twenty-four arms (rays). Star sapphires and rubies generally have six arms, which lie parallel to the crystallographic axes. Other gemstones that may show stars when cut as cabochon include garnet, quartz, and spinel.

Where the inclusions or cavities are not present in large enough concentrations to form a star, they may be seen as silk, with the light reflected from patches of parallel inclusions, such as can often be seen in sapphires.

REFRACTIVE INDEX

Refractive Index, Birefringence, And The Refractometer

Light that enters a gemstone is refracted (bent) from its original path in air as it enters the denser medium. Cubic and non-crystalline materials are singly refractive, which means that the light is refracted equally in all directions. Gemstones from the other crystal systems are doubly refractive. In doubly refractive gemstones, the light enter­ing the gem is split into two rays of light; each is slowed and refract­ed by a different amount. Where the difference is large, such as in calcite, double refraction can be seen as a double image through the gemstone. In sphalerite, a doubling of the image of the back facets (pavilion facets) can be seen when viewing them through the front of the stone (crown facets).

There is a mathematical relationship between the angle at which light strikes a gemstone and the angle of refraction, from which the refractive index (Rl) of a gemstone can be cal­culated. The refractive index of most gems can be measured accurately using a refractometer or Brewster Angle Meter and can be used to help identify the stone. Singly refractive gemstones have a single refractive index. Doubly refractive gemstones have a range of refractive indices; the difference between the maximum and minimum values is the birefringence (DR.).

DISPERSION IN GEMOLOGY

Sunlight {white light) is made up of the colours of the rainbow (spectral colours). Each colour corresponds to a different wavelength of light and a different energy, and each is refracted (bent) to a different degree (measured as its refractive index) as sunlight shines through the rain, effectively separating and spreading the colours to form the rainbow (dispersion). In a gemstone such as a diamond, which has a high dispersion, flashes of rainbow colours appear to come out of the stone as the gem or its light source is moved.

The colours of the rainbow are the colours of the visible spectrum, the wavelengths of light with the energies we are able to see. On either side of the visible spectrum other energies such as X-rays, ultra-violet rays and infrared rays interact with the crystal structure to modify the colour or produce optical effects such as fluorescence and phosphorescence.

THE IMPORTANCE OF LIGHT IN GEMOLOGY

Colour is the most obvious optical property, but the way that light interacts with the structure of a gemstone also gives it other optical properties which combine to make each gemstone unique. Effects produced by light reflecting off a gem's surface, off layers or struc­tures within or passing through a gemstone assist the gemologist in identification, but also give each gemstone its particular quality and beauty.

Light reflected off the surface of a gemstone gives the gemstone its lustre, whether it looks metallic, glassy or dull. The brightness and quality of the lustre depends on the condition of the surface, the degree of polish and the refractive index. The greater the amount of light that is reflected back towards the eye, both from the surface of a gemstone and having entered the stone, from internal reflections such as off the inside of the back of the stone (pavilion facets), the brighter the stone will appear. The amount of light, which can pass through a gemstone, will define whether it is transparent, translucent or opaque.

CRYSTAL SYSTEMS IN GEMOLOGY

There are seven crystal systems based on their crystal symmetry and crystallographic axes (length and direction) and all crystal structures can be assigned to one of these systems.

Each of the crystal systems, except triclinic, has horizontal axes and a vertical crystallographic axis. The direction and length of each axis and the angles between them (alpha, beta, and gamma) are defined by the crystal structure (lattice).

Cubic- four three fold axes- three axes mutually at right angles, and of equal length.

Tetragonal – one vertical four fold axis- three axes mutually at right angles; one axis conventially held vertically, differing in length from the other two.

Orthorhombic- either one two fold axis at the intersection of two mutually perpendicular planes; or three mutually perpendicular two fold axes- three axes mutually at right angles, all of different length.

Monoclonic- one two fold axis- three axes of unequal length; two axes are not at right angles to the plane containing the other two.

Triclinic- either a centre of symmetry or no symmetry- three axes, all of unequal length, none at right angles to the plane containing the other two.

Hexagonal- one vertical six fold axis- four axes, three of equal length, arranged in a horizontal plane; the fourth perpendicular to this plane and of different length from the other three.

Trigonal- one vertical three fold axis- same as for hexagonal.

CRYSTAL SYMMETRY IN GEMOLOGY

Crystals can be classified into groups depending upon their symmetry, which in turn is defined by the degree of regularity in the arrangement of the atoms in the crystal structure. This arrangement also affects their optical and physical properties, for example how they react to light, the way they look and feel, and their hardness.

The main method by which a gemologist usually defines the symmetry of a crystal is by reference to its planes of symmetry and axes of symmetry (sometimes referred to as mirror symmetry and rotational symmetry).

plane of symmetry

A plane of symmetry is an imaginary plane (or mirror) that divides a crystal such that the image on one side of the plane is the mirror image of that on the other side.

AXIS OF SYMMETRY

An axis of symmetry is an imaginary line (or thread) that runs through a crystal and about which the crystal can be rotated, in such a way that it looks the same two, three, four, or six times during a complete circle (rotation) of 360 degrees.

crystallographic axis (reference axis)

A crystal is also defined by its reference or crystallographic axes. The crystallographic axis is an imaginary line that runs through a crystal and indicates both the direction and length of the repeating pattern of the atoms (the lattice structure). The lengths of the crystallographic axes are proportional to the repeat directions in the three principal directions (a, b, and c) in the crystal structure or lattice (x, y, and z). Crystallographic axes are not the same as the axes of symmetry, although they may run parallel to each other.

CRYSTAL STRUCTURE

Each gemstone group or variety has a unique chemical composition that describes its essential elements or ingredients. Each gemstone is made up of atoms, the smallest building blocks of the structure. Where the atoms are constituted in a regular and repeating three-dimensional pattern, they are said to possess crystal structure and are referred to as crystalline.

amorphous

Where there is no crystal structure and atoms are randomly arranged, the material is said to be amorphous (from amorph meaning without shape or without form). obsidian (volcanic glass) is an example of an amorphous material that can be used as a gemstone.

crystalline

Most natural and artificial gemstones are crystalline. Crystalline materials have directional properties, both physical and optical, that are as a result of the three-dimensional arrangement of atoms and the type of bonds between them. As the gemstone forms, it grows by adding layers of atoms. The many ways that the atoms can com­bine is reflected in the many different crystal shapes including those with flat faces and sharp edges, needle-like points, or with smooth and rounded surfaces.

Although the internal crystal structure of the crystalline gemstone may define its external shape (habit), this is not always the case. Con­ditions during formation may be such that an irregular shaped crystal forms, or a crystal may have been broken, cut or polished. Whatever the final shape of the crystal, the internal structure is still the same, it is still crystalline.

twinning

The crystal structure may have parts that are reflected, repeated incorrectly or rotated, which may result in the creation of a twin crystal. At the junction of the parts of the twin, the change in orientation of the crystal structure (lattice) and direction of growth may be seen as a re-entrant angle at the surface. Repeated twinning (lamellar twinning) can sometimes be seen as layers in gemstones. Twinning of a crystal structure may be due to changes in temperature or pressure during or after formation.

POLYMORPH

Occasionally the chemical composition is such that it can form more than one crystal structure. Each structure is called a polymorph (from 'poly meaning 'many' or 'more than one', and morph meaning 'shape'). Which of the polymorphs forms will depend upon factors such as conditions during formation (for instance, temperature or pressure)? diamond and graphite, for example, are formed from the element carbon, but the carbon atoms have bonded in different ways, forming different crystal structures in each of the minerals.

ISOMORPH

Some gemstones have a range of chemical compositions, but have the same crystal structure. They are called isomorphs. An example is the garnet group. At opposite ends of the range are the two end members and between these are the members of the isomorphous series. almandine and pyrope are respectively the- iron-rich (Fe) and the magnesium-rich (Mg) end members of the garnet family.

cryptocrystalline

Crystals composed of many crystal structures that have grown together are termed polycrystalline. Where the crystal structure is too small to be seen with the eye and a microscope is needed, the crystals are termed microcrystalline or cryptocrystalline (from the Greek crypto meaning 'hidden').

Mining Of Gem Stones

A study of gemstone mining and retrieval covers every mining method, from the ancient traditional searches in streams and rivers using just a pan or sieve, to the ultra-high technology and research used in diamond mines deep underground. Ultimately, any source of gemstones will only be mined or exploited if the source is viable. It must be possible to mine or retrieve the gems at a profit to make the venture a viable business.

The oldest and most traditional methods are still used in areas where gemstones are near the surface, relatively easy to find and retrieve, and where labour is cheap and affordable. For example, in South Asia, local inhabitants search rivers and streams, as well as the gravels and sediments that were once ancient riverbeds or streams. They use basic equipment such as baskets, sievepans or buckets to retrieve river gravels and sediments, and a sieve or pan to begin to separate the gemstones.

This is possiblebecause the gemstones are generally heavier than the surrounding mud, pebbles, or rock fragments. As a pan of water and sediment is 'jiggled', the gemstones settle in the pan as the lighter constituents and the water are washed over the pan's edge. The heav­ier concentrate may be sieved to separate the larger gems, or spread out on tables or cloths to be hand-sorted and the gems found by eye. These gemstone localities, found associated with the sedimentary rocks of rivers and streams, are called 'alluvial deposits' or placer deposits. They are secondary deposits: they are not found in the rock in which they were formed, but where they have been transported to as a result of weathering and erosion.

Gemstones that survive the journey tend to be those that are sufficiently hard to withstand the conditions without breaking rather than those that are heavily includ­ed, that fracture easily, or break along cleavage planes. They are generally harder and heavier than surrounding minerals and in water they tend to sink faster and are therefore not carried as far. The surviving gemstones will generally become concentrated in pockets or areas along the riverbanks or within sediments, as gem gravels.

Alluvial deposits, such as the gem gravels of Sri Lanka and Burma (Myanmar) contain a diverse range of gemstones including RUBY, SPINEL, SAPPHIRE, TOPAZ, TOURMALINE, CHRYSOBERYL, and GARNET. Gem grav­els may contain good crystals, such as diamond, ruby and spinel, but generally the gemstones show signs of wear and tear. Crystals may be fragmented and rounded, and the surface may be scratched or frosted due to fric­tion. Because nature has already sorted out the weaker specimens, the percent­age of gem-quality stones within gem gravels is high, making their retrieval worthwhile.

Because a number of gemstones that are associated with gem gravels are often found together, the discov­ery of one type of gemstone from a gem 'association' can be used by explo­ration teams and prospectors to 'home in on' or 'trace' potential gemstone mining areas. Another technique is to map the courses of ancient river beds or present-day rivers and streams, and follow tracer gems downstream in the hope of finding areas where the gemstones are in sufficiently large enough concentrations to be retrieved.

In Colombia, emeralds form within thin layers of white limestone in soft, black carbonaceous shales. Enormous trucks are used to transport the soft shales to washing plants, where the emeralds are retrieved by separating the harder limestone from the soft shale. Landscapes are altered as hillsides are removed. In addition to the large-scale removal of the rocks, there may also be groups of people working on a much smaller scale. As the remaining shales are weathered, the gems may be washed out and transported downhill into the valleys below. Local people search the river beds and sediments for emeralds, often just using spades or their bare hands.

Where the rock is harder, these methods are insufficient. For example, rubies can form in metamorphic rocks. In some instances, picks and drills may be sufficient to price loose the gemstones from the parent rock (the host), while in others the rock has to be mined, crushed, washed, and sorted to retrieve the gemstones.

Diamonds are mined on a larger scale and with more highly mechanized methods than any other gemstone, because of their value and range of uses. The better quality stones may be fashioned as cut gems, while even the non-gem-quality diamond can be used as an abrasive or for other industrial purposes. The percentage of gem-quality to industrial-quality diamond varies from mine to mine and from continent to continent.

On the other hand, pipe mining of igneous rock (kimberlite or lamproite) pro­duces a greater total yield of diamonds, but typically only 20-25% are of gem quality.

The diamond pipes are mined from the surface, removing the rock and transporting it to processing plants to be crushed and washed before removing the diamonds. As the pipe is dug out, a large pit is formed. Once a pit is about 300 metres (1,000 feet) deep, underground tunnels and shafts are needed to excavate and remove the diamond-bearing rock.