This red variety of corundum occurs as bright red, purplish or brownish- red and deep pinkish- red colors.
Ruby is the birthstone for July.
One sixteenth-century writer wrote of ruby that ruby “gave control of the passions, drove out evil thoughts, secured possessions to their rightful owner, reconciled quarrels, brought peace and concord and also preserved bodily strength and health.”
In the past most red stones including ruby, spinel and garnet showing a good hardness and luster were often called ‘ruby’. As our knowledge of gems improved these other materials became better recognized in their own right, but misidentification is still common.
Ruby is heated to improve the quality of its color. Heating can, for example, remove the brown tones from Thai rubies and the blue tones from stones from Vietnam or Mong Hsu.
As a result of the heat treatment process, fractures and cavities may be filled with an artificial material. This material is an amorphous mixture of the molten powdered flux in which the rubies are placed for heating, plus material present in and around the original rough ruby. These glassy residues may be evident on the polished surface of the faceted gemstones as areas of lower luster in contrast to the higher luster of the surrounding host ruby. These glassy residues within the healed fractures can have the appearance of flux-filled fractures seen in flux melt synthetic rubies. This glass filling of rubies is also done intentionally to improve the appearance of the stone by hiding fissures and filling cavities and is also carried out using lead glass. This filler gives blue flashes as the stone is rotated and moved and gas bubbles may be trapped in the glass filler.
Red oil may also be used to reduce the visibility of fractures and to improve the color of a ruby.
Localities and inclusionsRuby is found in commercial quantities in many locations:
|Locality||Comments and inclusions|
|Myanmar (Burma)||Fine- quality rubies of good color are found in the Mogok district. These stones commonly contain short, fine retile needle inclusions known as silk. A variety of other mineral inclusions are also seen, particularly corundum, calcite, dolomite, spinel, zircon, garnet, apatite, graphite, pargasite and yellowish sphene crystals. Inclusions may be well-formed, or corroded, rounded crystals. The colour is often in swiris which have the appearance of treacle. Intersecting twinning planes are often seen. Rubies from Mong Hsu show central blue ‘cores’ in their untreated state, and whitish clouds of rutile; these stones are usually of a lower quality than those from the Mogok district.|
|Pakistan||Similar to those from Myanmar. Good colour, but clarity generally poor so mostly cut as cabochons.|
|Afghanistan||Usually only in small sizes, rarely producing stones in excess of 2 ct. Stones often contain blue patches similar to Vietnamese stones; also calcite, mica and rutile inclusions.|
|Thailand (Siam)||These rubies are naturally darker or more brownish-red than Myanmar stones. They often have irregularly-shaped fluid inclusions with dark crystals at their centres. Intersecting twin planes are also often seen. In heat-treated stones the colour can be very similar to that of Myanmar stones. Fluorescence is weak, due to iron content.|
|Sri Lanka (Ceylon)||Shades of red and pink. Stones often contain long coarse rutile needles, biotite mica, pyrite, metamict zircon grains with tension haloes, pronounced hexagonal colour zoning elongate negative crystals or cavities containing fluid and/ or crystals.|
|Tanzania||Stones from Logido are often of fine colour and are found in a bright green chrome zoisite rock. This combination of ruby and green zoisite is used as an ornamental stone. The ruby is usually cut as cabochons. Facetable material occurs in the Umba valley often showing twinning planes and apatite crystals. Star material is found at Morogoro.|
|Vietnam||Fine colour, good clarity, similar to Myanmar rubies. Much material contains blue patches, which may be removed or modified by heating.|
Rubies are also found in many other localities including Australia, Kenya, Namibia, Madagascar, India, USA, Russia, China and Nepal.
Although these inclusions are frequently seen in gemstones from the areas indicated, they also occur in stones from other localities.
Material with a similar appearanceRed glass, red tourmaline (sometimes called rubellite in the trade), almandinepyrope garnet-topped doublet, and corundum-corundum doublet.
TestsDichroscope- Moderate to weak, red and oranges-red and dichotic.
Polaris cope and refract meter – Spinel and garnet are isotropic; the polariscope will reveal the singly refractive nature of spinel, but almandine usually shows anomalous extinction effects. If faceted, ruby is readily distinguished by its RI and birefringence.
CCF— Rubies, pink and purple sapphires contain chromium and therefore emit a red glow when viewed through a CCF as will most spinel; almandine will appear dark.
UV — They also fluoresce red in UV light. Natural rubies and purple sapphires often contain traces of iron, and this may sometimes mask their fluorescence.
Spectrum— The colour is caused by the presence of chromium, and the full diagnostic spectrum consists of a double line plus two weaker lines in the red, general absorption of the yellow and green, three fine lines in the blue, and general absorption of the violet. With a hand spectroscope only two of the lines in the blue are normally seen. A bright emission line may be seen in the red part of the spectrum. The spectroscope is useful to help distinguish red stones, particularly if they are small and set in jewellery. The garnet and ruby can be distinguished by their absorption spectra. The absorption spectrum of red spinel resembles that of ruby, but there are significant differences.
Quality FactorsThree important quality factors that affect value are colour, clarity and quality of cut. The highest value stones have a good rich colour, they are relatively free from inclusions and have a bright, lively appearance. In the series below, the stones are in order of value per carat, with the highest value on the left.
This pale yellow, honey-coloured or green/brown translucent to transparent variety showing good chatoyancy was at one time called cymophane. Chatoyancy is caused by very fine parallel needles or hollow tubes. Good examples show a very sharp bright ‘eye’. To display these cat’s-eyes to best effect these stones are cut as cabochons.
Cat’s-eyes are found in brazil and sri Lanka. Other localities include India, Myanmar (Burma) and Zimbabwe.
Materials with a similar appearanceCat’s-eye quartz; bleached tiger’s-eye quartz cut to show cat’s-eye, fiber optic glass and yellow to brown cat’s-eye opal.
Corundum occurs in all colours. Sapphire is the name given to all colours of corundum other than red. When used on its own, the name implies blue sapphire. Blue Sapphire is coloured by a combination of iron and titanium. Some sapphires may exhibit a colour change.
Both heat treatment and diffusion treatment are common in sapphires.
Materials with a similar appearanceBlue glass, iolite, blue tourmaline, tanzanite, blue spinel, blue synthetic spinel and corundum-corundum doublet.
Localities and inclusions
|Locality||Comments and inclusions|
|Kashmir, India||Restricted production of fine-coloured blue sapphire, often with a very slight milkiness caused by very small inclusions; colour zoning crystals, stress fractures, negative crystals.|
|Myanmar (Burma)||Sapphires of good colour but sometimes rather dark. Inclusions may be long needles of rutile, apatite. Convoluted feathers, silk, hexagonal colour zoning present in some stones.|
|Thailand (siam)||Sapphire of poor to good colour. Straight and angular colour zones with silk, feldspar, hornblende, pyrrhotite, spinel, uranium pyrochlore and secondary fluid inclusions.|
|Sri Lanka (Ceylon)||An important source of good-quality sapphires, from very dark blue to very pale, some strongly particoloured. Also pink, yellow and pink-orange ‘padparadscha’. Crystal inclusions: particularly mica, pyrite, zircon crystals with haloes; elongate negative crystals; healed fractures often resembling fingerprints; rutile ‘silk’, two-phase inclusions.|
|Kampuchea (Cambodia)||Medium to good colour, in some cases similar to Thai stones. Small red crystals of uranium pyrochlore, plagioclase feldspar, crystals surrounded by healing fractures.|
|USA (Montana)||Untreated blue stones are usually very bright in appearance with an almost metallic lustre, frequently violet in tungsten light. Small well-formed crystals such as garnet, rutile, calcite and pyrite. Hexagonal zoning. Also pale green, yellow and pink stones.|
|Australia||Good to dark blue, dark greenish-blue and almost black sapphires. Yellow, green and parti-coloured stones also common. Strong zoning, feldspar, zircon crystals with associated haloes.|
Sapphires are also found in many other localities including Chine, Nigeria, Kenya, Tanzania, South Africa, Mozambique and Madagascar.
TestsDichroscope- Two different shades of blue. Although, some blue sapphires show blue plus green. Dichroic.
Spectrum—The absorption spectrum of blue sapphire often shows a single band in the blue. In some natural stones, absorption consists of a group of two or three close bands in the blue; a similar spectrum is seen in may green and golden stones.
Sapphire is the birthstone for September. In the middle ages it was said of sapphire that it preserved chastity, discovered fraud and treachery, protected from poison, plague, fever and skin dieases.
Quality FactorsThree important quality factors that affect value are colour, clarity and quality of cut. The highest value stones have a good rich colour, they are relatively free from inclusions and have a bright, lively appearance.
All colours of corundum excepts for red and blue are called fancy-coloured sapphires. These fancy-coloured stones should be described by colour, for example, ‘green sapphire’, ‘yellow sapphire’ and ‘purple sapphire’. The only exception to this is the rare pinkish-orange stones called padarsdscha.
Common TreatmentsThe irradiation of colourless to pale yellow sapphire produces a deep yellow to brown colour which can fade rapidly on exposure to heat or light. Treated yellow sapphire is difficult to distinguish from natural yellow sapphire.
TestsDichroscope- Strong in varieties other than yellow. Other colours will show differing shades of the body colour. Corundum is dichroic.
UV — Almost colourless, yellow and golden sapphires from Sri Lanka usually glow apricot or orange under LWUV light; they do not usually show an iron spectrum, which distinguishes them from similar-coloured stones found in Australia which do not fluoresce but show a typical sapphire iron spectrum.
Spectrum— Green and golden stones can have similar spectra to blue sapphires.
Quality FactorsFor fancy coloured sapphires, the factors that will influence the value of the stone are the depth of colour, clarity and appearance. However, another important factor is the colour itself, for example these five fancy sapphires have been placed in value order according to one UK stone dealer. The natural coloured padaradscha on the left being the most expensive. The yellow, although a nice bright stone, is the least as it is a more common colour than some of the others in this group, so has a lower price per carat. The effect of colour on price will also be dependent on fashion and local demand.
Colour and varietiesColour-body colours of pearl can be broadly classified as white, golden, grey, brown(Chocolate) and black. In addition, there may be various iridescent tints including pink, green, blue and lilac contributing to the overall colour of the pearl.
Pearls used in jewellery fall into two categories:-
Natural- produced entirely by natural categories Cultured- Artificially initiated and farmed.
- Pearls may be:
- Marine (saltwater)
Pearls are among the earliest of gem materials used for personal adornment. Created by soft-bodied animals which secrete a hard outer shell of calcium carbonate around themselves and live in fresh- and salt-water locations throughout the world, pearls need no cutting to reveal their beauty.Lustre- the unique lustre of pearl is a complex effect, quite different from the lustre of polished gemstones. It is produced partly by the surface reflection of light and partly by a subtle iridescence. The resulting combined reflection effect is known as the ‘orient’ of pearl.
Weight- a unit of weight common used for natural pearls is the pearl grain: 1 pearl grain = 0.25ct. Momme is a unit of weight used for cultured pearls: 1 momme= 3.75g
Pearl is one of the birthstones for June.
As an emblem of modesty, chastity and purity, the pearl symbolizes beauty, love, success, and happiness. They often symbolize a happy marriage and in many countries are used as a wedding gift.
The origin of pearlsPearls are produced in water-dwelling animals, in certain types of shellfish known as molluscs. The most important pearl-producing molluscs are saltwater oysters and freashwater mussels. These animals belong to a subgroup of the molluscs, known as bivalves, in which the shell consists of two hinged parts. Pearls can also occur in gastropods such as the rare conch pearls—these will be coverd in the IDT Graduate Gemologist Diploma Course.
The familiar round pearl forms inside the body of a mollusc; these are called cyst pearls. Blister pearls form on the inside of the shell and need to be sawn away from the shell to release them. The unique lustre of a pearl is created by the nacre layers that cover the pearl. The nacre layers are deposited by the mantle of the oyster; this is the soft tissue of a mollusc that also creates the mother-of-pearl inner surface of the shell. In natural pearls, pearl formation may be related to damage of the mantle caused by parasites, disease or attack by predators. In cultured pearls, the process involves using a section of the mantle of another oyster with or without a bead nucleus; this is called tissue nucleation. More information about the formation of pearls and the different molluscs involved is discussed in the IDT Graduate Gemologist Diploma course.
Natural pearlsNaural pearls possess a concentric structure made up of layers of nacre. These layers are affected by various factors including seasonal growth, temperature change and food supply.
LocalitiesSaltwater (or marine) natural pearls can occur in a wide range of localities. In the past the most important natural pearl fisheries were in the Central Americas, the Persian Gulf and the Gulf of Manaar (between Sri Lanka and India). Commercial fishing for natural pearls is now virtually nonexistent due to fundamental changes in ocean ecosystems and to economic circumstances. Although, natural pearls are still occasionally found in these areas, as well as in oysters in the area farmed for the pacific ‘South Sea’ cultured pearl; other localities include the Red Sea, the Gulf of Mexico and off the coast of Venezuela. The rarity of natural pearls often means that necklaces are graduated because of the difficulty in matching their sizes.
Other natural marine pearls such as the non-nacreous conch pearl are covered in the IDT Graduate Gemologist Diploma Course. Natural freshwater pearls were fished from rivers in most European countries, but little or no fishing is now carried out although natural freshwater pearls may still be found in Scotland, Letland and North America.
Cultured pearlsThere is now an enormous market in cultured freshwater and saltwater pearls. The insertion of objects such as figures of Buddha between the shell and mantle of molluscs, in order to obtain a covering of nacre, has been practised by the Chinese since the thirteenth century. Although, the process of obtaining cultured blister pearls was not patented until 1896 by Kokichi Mikimoto in Japan. Round cultured pearls from Japan first appeared on the market around 1905.
- Gem garnets occur in various colours including red, orange yellow and green.
- Rarely four-and six-rayed star stones can occur.
- Colour-change garnet which appears bluish-green in daylight to purplish-red in incandescent light.
Colour and varieties
Gem garnets are transparent to translucent and have a bright vitreous lustre. The name ‘garnet’ refers to a group of minerals related in composition and crystal structure. As with other gemstones, quality determines whether garnets are faceted or cut as cabochons. Garnets have a mdium to high dispersion often masked by their body colours.
All garnets have the sme general chemical formula X3Y2(SiO4)3. The group is split into two isomorphous series:
- In one series X may be magnesium (Mg), iron (Fe) or manganese (Mn), while Y is always aluminium (Al).
- In the other series, X is always calcium (Ca), while Y may be chromium (Cr), aluminium (Al) or iron (Fe).
Occurrence and crystal habitGarnet occurs as crystals in metamorphic and igneous rocks, as rolled pebbles in alluvial deposits and broken fragments. Nearly all crystals are of rhombic dodecahedron or icositetrahedron form. Oriented fibrous inclusions may be seen.
Garnet is the birthstone for January. The name ‘garnet’ is derived from the Latin name granatum – meaning pomegranate because of the strong resemblance of the red crystals to the seeds of a pomegranate.
During the great biblical flood, Noah was said to have used the red glow from a garnet to guide his way through the darkness. Garnet is often associated with illumination and bringing light into the dark. For example, garnet jewellery was buried with Vikings to help light their way on the journey to Valhalla. Garnet is also said to have great healing properties; it could strengthen the heart and cure melancholy if ground up and swallowed. In the middle ages knights would place them on their shields to prevent injury.
General CareGarnets are usually very durable, except for demantoid which is the softest of all garnets; more care is needed when handling this material.
- Thermal shock as abrupt changes of temperature may cause fracturing
- Steam cleaners
- The jewellers torch.
- Precious Opal -- Opal with play-of-colour on a pale (white opal) or dark (black opal) background.
- Fire Opal— Transparent to translucent brwonish-yellow, to ornage, to reddish-Orange; may show a play-of-colour.
- Common Opal— Translucent to opaque material with a range of body colours but no play-of-colour.
Colour and Varieties
Opal can be divided into two varieties — precious opal and common opal. Both are fashioned as gemstones, but only precious opal shows a vivid play of iridescent colours for which this gem is prized. Fire opal is named for its colour and water opal for its appearance; both may show a play-of-colour.
Some white opal dispalys a translucent milky appearance and it is this, not the play-of-colour, which is properly termed ‘opalescence’.
Opal is one of the birthstones for october.
The Greeks thought opal gave powers of foresight and prophecy, while the Romans saw opal as a symbol of purity and hope. The Australian aborigines have a legend that opals were born when Earth’s Creator descended to Earth on a rainbow and where his foot first tourched the earth the stones around began to sparkle, displaying all the colours of the rainbow.
Occurrence and habitOpal is deposited in cavities and fissures in rocks from low temperature, silica-bearing water. Opal may also replace existing structures, such as fossils or soluble minerals.
General and workshop careOpal is soft so should be treated with particular care. Clean by wiping with a damp or dry cloth only.
- Doing anything that could cause abrasion, pressure or knocks.
- Heat and dry/hot storage conditions; where possible store with moist cotton balls or a dish of distilled water to avoid dehydration which can lead to cracking and crazing.
- Direct sunlight or other strong lights.
- Chemicals / acids and detergents, including perfumes, hairspray and make-up, do not soak in jewellery cleaner as opal is porous.
- Ultrasonic and steam cleaners
- The jewellers’ torch.
The Green colour of emerald is caused by traces of chromium (Cr), although vanadium (V) may also be present in some stones.
Trapiche emerald – trapiche emerald crystals from colombia are of several types and usually display a six-rayed radial structure. Commonly, they consist of a central hexagonal green crystal from which six segments of similar colours have grown outwards. A fine-grained mixture of colourless beryl and feldspar fills the radial spaces.
Emerald is the birhtstone for may.
In the 1914 book The Book of Talismans the author writes of emerald: “If worn in a ring, emerald strengthens the memory and protects from giddiness.” It was also said to guard sailors and fisherman from perils and mishaps at sea if suspended “round the neck so as to lie upon the chest”.
Durability and fashioningEmerald is a very brittle stone usually containing fissures and is easily chipped. It shoud never be put in an ultrasonic clearner, but washed in mild soapy water. It should not be left to soak in detergents of solvents. Although cut in various styles, a common cut for emerald is a rectangular step cut, with the corners truncated to give an eight-sided (octagonal) outline.
This has been used so extensively for emerald that it is often called the emerald cut. This brittleness can also influence the design and setting of emerald set jewellery.
General CareEmerald, can fracture easily. Avoid wearing whilst doing anything that could cause the stone to be knocked. Do not allow emeralds to come into contact with solvents as these can affect any fillers that may have been in the stone.
- Heat which can cause fracturing in included stones and the evaporation of any fillers
- Ultrasonic and steam cleaners.
Localities and inclusionsEmeralds are found in colombia, Brazil, India, Pakistan, Siberia, South Africa, Zambia and Zimbabwe. Almost all natural emeralds contain inclusions, and these are very important in distinguishing natural from synthetic emeralds and other green stones. However, some growth features such as straight colour zoning are seen in both natural and synthetic emeralds.
Some inclusions are common for particular localities.This, when combined with an assessment of RI and SG, may provide an indication of the country of origin (to be covered in more detail in the IDT Graduate Gemologist Diploma Course)
|Locality||Comments and inclusions|
|Colombia||Three-phase inclusions (liquid-filled cavity containing a crystal and a gas bubble).|
|India||Two-phase inclusions (liquid-filled cavity containing a gas bubble).|
|Zimbabwe||Tremolite (usually fibrous or needle- like a crystals).|
|Siberia||Needle-like crystals of actinolite.|
|Many Localities||Mica flakes, pyrite and calcite, and also colour zoning.|
Common treatementsMany commercial emeralds are treated with oil or polymer resin, which fills those fractures (cracks and fissures) which reach the surface. This hides the fractures and temporarily enhances the appearance by improving clarity and colour. Ultrasonic cleaning will usually remove oil, replacing it with dirty cleaning fluid which dries in place and spoils the stone.
The detection of oil is not easy. Microscopic examination may reveal interference colours in the fractures. Also fractures may not be completely filled by the oil, and gaps may be visible. Before cleaning a stone, look for traces of oil on the surface and for oil marks within the stone packet. Some oiled stones may have been treated with substances that fluoresce in UV light.
The detection of polymers is difficult because the RI of the polymer is very close to that of the emerald. Very careful examination of the surface under magnification may reveal the fracture. By rotating the stone a coloured flash of light may be seen reflected from the filled fracture. This can be orange, blue or another colour, depending on the filling.
DyeingEmerald is sometimes dyed to improve the colour. The treatment may be identified by the concentrations of dye in cracks in the material.
Materials with a similar appearanceSimilar looking material include green tourmaline, YAG, glass, aventurine quartz, dyed green quartz, GTD’s and soude stones. In facts almost any green material of similar colour has been used to imitate emerald either in its rough or cut form at some time. A common rough simulant is a composite green glass and blackish matrix material, however composites have also been made up using low-grade emeralds.
TestsDichroscope- Weak to strong—blue –green and yellow-green; dichroic.
UV and filters— some emeralds fluoresce pink or red under UV and appear pink or red through the CCF. This is not diagnostic as some emeralds, mainly from India and Africa, remain green due to traces of iron which quench the fluorescence.
Note: Although the CCF was first created to help identify emeralds, today it can be used as an indicator test only and further testing would be required.
Spectrum— A typical emerald spectrum has fine lines in the red, a weak general absorption in the yellow-green and a weak line in the blue.
Quality FactorsThe depth and tone of colour of emerald are very important with a rich soft green being most highly regarded. Most emeralds do have inclusions, many being heavily included. The better the clarity, the higher the value, but the clarity of emerald should not be compared to that of stones such as aquamarine. Locality is still a major price factor with most emeralds. The stones below are in order of value per carat, with the highest value on the left.
The Romans believed coral would prevent childhood ailments, as well as using it as a charm against sterility. It is a soft material and is commonly fashioned as cabochons, beads, earring drops, cameos and carvings. It is easily attacked by acids and contact with cosmetics should be avoided. To clean use mild soapy water.
The pink coralsb> are characterized by a fine corrugation structure running along the lengths of the branches which is often visible in fashioned pieces. Black and golden coral displays concentric structures in the branches, similar to the growth rings of a tree, and have a distinctive subsurface pimply appearance.
Common treatments-Precious coral may be stained, usually red or pink. Golden coral can be obtained by bleaching black coral.
Localities-Today the majority of precious coral is from the seas to the south of Japan. Black coral is found in the West Indies, off the Hawaiian Islands, Australia and in southern waters towards Antarctica. There are strict controls on the fishing, export and import of coral in most countries.
Colour and varieties
- Colourless (white); pale yellow, pale brown, pale green or grey.
- Fancy colours (those of a distinct colour) including yellow and brown; rarely green, pink, blue and black; very rarely red and purple.
Commercially, diamond is the most important of all gem species. It is estimated that diamonds account for approximately 90% of the value of gemstones purchased thoughout the world. Diamond is faceted to display its unique combination of adamantine lustre and fire. This fire is a direct result of the high disperaion (0.044) of diamond. Diamond displays a higher degree of dispersion than any other natural colourless gemstone.
A pure diamond is composed entirely of carbon and is colourless. However, diamond crystals usually contain trace elements, the most common of which is nitrogen; this causes the pale yellow colour often seen in natural diamonds. A much rarer trace element in diamond is boron which causes a blue colour.
However, not all diamonds are coloured by trace elements. Brown and pink diamonds owe their colour to defects in the crystal structure and green diamonds are the result of vacancies (holes) in the crystal structure. These causes of colour are covered more fully in the IDT Graduate Gemologist Diploma Course.
The alluvial deposits of India and Borneo were the only known sources of diamond from classical times until the eighteenth century. The important Brazilian fields were discovered in about 1725.
The alluvial and kimberlite pipe deposits of South Africa were discovered in the latter half of the nineteenth century, and those in Siberia during the 1950s. In the 1980s Australia became an important producer of diamonds and the most recent important commercial discovery was in Canada where mining began in the late 1990s.
The important gem diamond-producing countries today include Angola, Australia, Botswana, Brazil Canada, China, Congo, Namibia, Russia, Sierra, Leone, South Africa and Tanzania.
Diamond is the bithstone for April.
Many of the myths surrounding diamonds originated in India and have been passed on and adapted as diamonds reached Europe.
Ancient Hindu lapidaries warn against the wearing of flawed diamonds as this could bring misfortune, whereas: “He who has a pure body, and carries on his person a diamond that is sharp-pointed, without blemish and entirely flawless, shall daily increase his worth in happiness, prosperity, children, wealth, crops, cows and livestock, to the end of his life.”
In the West, most Medieval authors state that diamond has the power to protect against poisons, although others believed it to be a poison.
Diamonds have also been associated with love, but this image and their use in engagement rings has only become common place relatively recently. This is primarily due to the rarity of diamonds prior to the African finds in the late nineteenth century and a widespread advertising campaign started by De Beers in the late 1930s.
Occurrence and crystal habit
The octahedron form is a common habit for diamond. Some more complex forms also occur, including the dodecahedron, frequently with rounded faces, and the traingular twinned crystals called ‘macles’. Triangular pits – trigons – may be seen on octahedral faces.
Although diamonds are found in diamond-bearing volcanic pipes, they do not form within them. The diamonds form in the mantle and are captured by the molten magma as it drives its way up through the upper matle rocks capturing diamonds from the walls of the rocks through which it passes. The volcanic rocks in which diamonds occur are called kimberlite and lamproite. Although not all such rock contains diamond, other gemstones such as garnet and spinel may be present, and their occurrence is often a useful indicator of the possible presence of diamond.
The industry’s procedure for extracting diamonds is different from that used for all other gemstones. The object of mining the ‘coloured stones’ (i.e gems other than daimond) is to extract only the stones of gem quality, whereas all diamonds can be useful regardless of quality or size. Those diamonds not of gem quality, wheras all diamonds can be useful regardless of quality or size. Those diamonds not of gem quality, the majority in terms of quantity, are used in a huge variety of industrial applications. Consequently, diamonds are prospected for and extracted on a mcuh larger scale and in a far more systematic, mechanized way than any other gem materials.
Diamonds are mined either from volcanic bodies of kimberlite or similar rock, or from the river and beach gravels where they accumulated after erosion from these rocks. Since gem-quality diamonds fetch a much higher price than industrial grade diamonds, a deposit must contain a sufficiently high proportion of gem grade material to make it economically worth mining. In an average economic deposit, 100 tonnes (100 million grams) of kimberlite ore would contain about 25 carats (5 grams) of diamonds, of which just 5 Carats (1 gram) may be gem quality.
More information can be found in section 10 about diamond mining and procedures now in place to prevent the exportation of Conflict Diamonds.
Durability and fashioning
Diamond is the hardest known natural substance and is faceted to display its unique combination of adamantine lustre and fire. Its supreme hardness ensures a lasting precision of cut, unique among gemstones. Most cut diamonds display knife-sharp facet edges and absolutely flat facets.
Although diamond has a hardness of 10 on Moh’s scale, it lacks the toughness of some stones due to its cleavage. The perfect octahedral cleavage in diamond may be exploited in the fashioning of diamond to split large crystals or trim off flawed material. Cleavage is seen in and on cut and rough stones.
The hardness of diamond varies according to the crystallographic orientation. But, in any direction, diamond is still much harder than any other gemstone. The hardness of diamond requries the application of special techniques in the cutting process, this along with the precision, in terms of proportions and angles, with which the stones must be cut demands a high degree of skill from the diamond cutter. Diamond is normally faceted but very occasionally uncut crystals are set.
The most popular cut for diamond is the round brilliant. The 57 facets (58 with the culet) of the well-proportioned brilliant cut are polished at specific angles relative to one another. This cut is designed to show the optical effects of brilliance (reflection) and dispersion to their best advantage. Diamond manufacture involves four basic stages: designing, cleaving or sawing, bruting and polishing.
In designing the brilliant cut, facets are angled to maximize brilliance and fire. The angle between pavilion facets and the girdle is the most important; if the pavilion depth of the stone is either too deep or too shallow then light is not returned to the eye reducing the brilliance of the stone. Changing the crown angle, or height, and thus the table size will alter the balance of brilliance and fire seen in the stone. See section 11.
Many other cuts are also used for diamonds including other brilliant cuts such as the square, pear, marquise and oval. The emerald and step cuts are also used. Baguettes and tapered baguettes are modifications of the step cut, typically used for small stones. Older styles of cut include the rose cut with a flat base and a dome of triangular facets; this style of cut is found mainly in antique jewellery. New desighns in diamond manugacture frequently appear, with a drive towards increased variety and choice for the customer and increased yield from the rough.
General CareDiamonds attract grease so avoid applying hand creams or soaps whilst wearing. Although hard, diamond possesses perfect cleavage: if knocked in the wrong direction it can cleave/fracture, so avoid sharp knocks.
- Burning; if using heat first coat the stones to protect against oxidizing conditions
- The jewellers’ torch, particularly if the stone has been filled.
- Ultrasonic and steam cleaners if the stone is heavily fractured or lead-glass filled.