顶级鉴定机构SSEF宝石课程：钻石（英文）
「Introduction to Diamonds」
「What are Diamonds?」
Diamonds: Gemmology & Mineralogy
In 1793, French chemist Antoine Laurent de Lavoisier first discovered that diamond is essentially formed of carbon. In diamonds, these carbon atoms are bound together in an extremely compact manner. A very much appreciated consequence of this high atomic density is diamond’s high refractive index (RI of ca 2.42). Indeed, diamond’s atomic density is so high that the speed of light entering it is effectively slowed: hence it has a high refractive index and a high dispersion of light, commonly referred to in gemstones as 'fire’.
When transparent, a diamond may take any colour of the rainbow, from red to orange, yellow, green, blue, purple, and any combination of these hues. White (like milk) diamond exists; it is translucent. Black diamond is opaque.
The World Jewellery Confederation (CIBJO) defines diamond as “a mineral consisting essentially of carbon that crystallises in the isometric (cubic) crystal system.” As such, synthetic diamond (produced by man) must be correctly disclosed and cannot be called 'diamond’ without the prefix describing its origin: synthetic, laboratory-created, or laboratory-grown.
Diamond is unique. Not only because it is the hardest mineral, nor because it is chemically inert, nor for its high refractive index; diamond is unique because it is the only material to combine all these exceptional chemical, physical, mechanical, and optical properties.
Sawing a rough diamond octahedron.
Diamond cutting workshop in Mumbai. More than 90% of the world’s diamonds are cut in India. Photos: Laurent Cartier, SSEF.
Diamond Hardness: Implications for Cutting
Diamond is the hardest mineral found in nature. As such, only a diamond is hard enough to cut and polish another diamond. This is possible because the different crystal faces of a diamond have different hardness – this is called hardness anisotropy.
Differential hardness in a diamond. The orientation of a diamond is critical when cutting it. The image on the right shows a rough diamond octahedron being sawed. Illustration & photo: SSEF.
A selection of different shapes and cuts of diamond gemstones below (Photos: SSEF).
「What are Diamonds?」
Cutting of Fancy-Coloured Diamonds
Commonly, diamonds are most appreciated if they show no colour at all. These diamonds owe their visual beauty mostly to their high brilliance, their pure white colour, and their well-defined and classical pattern of facet reflections, superposed by a few colourful sparkles due to dispersion of white light into its spectral colours within the diamond. The standard round brilliant cut is a classic example of how such colourless diamonds are presented.
For coloured diamonds, the cutting always aims to create the most intense colour saturation when looking at the stone. This is commonly achieved by using far more complex cutting styles with a high number of facets. These facets try to trap the light as much as possible in the stone through multiple internal reflections, with the result that the observed colour is more saturated. Such an effect is best seen in diamonds which are cut with corners (such as cushion, marquise, pear, or drop shapes) in their form.
Colour Grading of Fancy-Coloured Diamonds
The colour grading of diamonds is a very detailed process, as there is an interplay of three optical parameters which have to be taken into account to ultimately conclude on a colour grade: 1) colour hue (eg pink, blue, green), 2) colour saturation (from weak to strong), and 3) tone (from light to dark).
The interplay of these parameters may result in a pure colour, as for fancy pink diamonds, or in mixed colours such as 'brownish pink’, 'orangey pink’, or 'greyish pink’.
Adding further complexity to this issue is that the pattern of reflections may result in distinct differences in the visible colour saturation within certain parts of a diamond, thus making colour grading quite a challenge.
Coloured diamonds are graded 'table up’, whereas colourless diamonds are always graded 'table down’ (or from the backside), Thus, coloured diamonds are judged based on the best colour present when looking onto the stone from above.
The colour of coloured diamonds is graded on the part with the highest saturation. On the other hand, colourless diamonds are graded in the portion of the diamond with the least internal reflection. An experienced gemmologist will therefore always slightly tilt the coloured diamond in different orientations to fully appreciate and judge its colour and saturation.
Unlike for colour grading of colourless diamonds, there is currently no worldwide standard based on master diamonds of defined colour to grade coloured diamonds. At the Swiss Gemmological Institute SSEF, coloured diamonds are graded using a series of fancy-coloured master diamonds as internal standards split into the following categories (using pink as an example):
· pink (faint colour)
· pink (very light colour)
· pink (light colour)
· pink (fancy light colour)
· pink (fancy colour)
· pink (fancy intense colour)
「History of Diamonds」
India, and to a lesser extent Borneo, were the only source of diamonds until about 1730, when Brazilian deposits were discovered. In India, diamond was for thousands of years known as a symbol of power and religion. Most Indian diamonds were mined in alluvial deposits along rivers over many centuries.
Diamond was also highly regarded as a symbol of strength and wisdom in Buddhism. The Ratnapariksha of Buddhabhatta, a 6th-century Sanskrit lapidary treatise states: “[The one] who wears a diamond is protected against all dangers, be it by serpents, fire, poison, sickness, thieves, or evil spirits.”
The legends and the mysticism connected to diamonds were appreciated during the Roman Empire. With its fall, and the rise of Christianity in Europe, the situation changed and the appreciation for diamonds was greatly reduced for several centuries.
Another reason was the rise of Persia and Islam. As Middle Eastern states gained control over much of the trade with India, their rulers, who had a taste for lavish ornaments, took control of many of the larger and more attractive diamonds.
Once it became possible in Christian Europe to entertain ideas from the classical and Eastern traditions without the fear of being burned at the stake for heresy, the virtues of diamonds began to be recognised. Beginning in the 14th century, diamonds began to exert their power in the new Western culture of the Renaissance.
In the 18th century, when the Indian diamond mines declined, new and important diamond deposits were found in the states of Mato Grosso, Minas Gerais, Roraima, and Bahia in Brazil. After 1860, Brazilian diamond production rapidly declined, resulting in a great shortage of rough diamonds in the European cutting centres. After the discovery of South African deposits in 1870, European production nearly ceased.
South Africa was a powerhouse of diamond production, and deposits were discovered in other countries on the African continent during the 20th century. A lot of the mining and trading activities were linked to De Beers and its associated trading arms Diamond Trading Company (DTC) and Central Selling Organisation (CSO).
Ekati Diamond Mine in northern Canada. Photo source: Wikimedia Commons
「Famous Diamonds」
Historic Diamonds
The Beau Sancy
The first royal owner of the Beau Sancy was Marie de Médicis (second spouse of King Henry IV of France) who, after the King passed away, became regent of the Kingdom of France in 1610. In 1604, Marie de Médicis acquired the Beau Sancy from a diplomat and financier named Nicholas Harvey de Sancy – hence the diamond’s name. The Beau Sancy is also known in France as the 'Petit Sancy’, as there is another diamond called the Sancy, which weighs 55.23 carats and is to be found in the collection of the Louvre in Paris.
Following her death, the Beau Sancy was given to the Orange-Nassau family of Holland, then it was inherited by Frederick I King of Prussia. It remained part of the crown jewels of Prussia until the abdication of William II in November 1918. It was the property of SKH Prince Georg Friedrich von Preussen until May 2012, when it was sold to an anonymous buyer at a Sotheby’s Geneva auction for ca. US$ 9.57 million.
The Wittelsbach
The 35.56 ct Wittelsbach Diamond studied by the SSEF in 2008. The central image shows an exceptional example of 'tatami strain’ pattern. This pattern is an anomalous birefringence feature that is typical of type II diamonds and appears when the diamond is observed in polarised light. Photos: SSEF.
The Dresden Green
The Dresden Green diamond is a 41 ct diamond that was mined in India and brought to Europe at the beginning of the 18th century. It is one of the most famous coloured diamonds in the world, and an exceptional example of a naturally green-coloured diamond. August III of Saxony bought the diamond in 1741 and had it mounted. It was remounted in 1768 into a new design of a hat brooch in which it is still found today. The Dresden Green diamond was named after the capital of the German state of Saxony and can be seen in the historic Green Vault of the Staatliche Kunstsammlungen in Dresden.
Other famous historic diamonds include the Cullinan, the Hope, the Regent, the Koh-i-Noor, the Orlov, and the Centenary.
Left: The 105.6 ct Koh-i-Noor diamond in the British crown jewels. Photo: Wikimedia commons. Centre: The 189.62 ct Orlov diamond in the Imperial Sceptre. Photo: Gokhran. Right: The Centenary diamond rough (599 carats) and cut (273.85 carats). Photo: DTC.
Diamond Records at Auction
The Oppenheimer Blue is a 14.62 ct vivid blue diamond that, in May 2016, became the most expensive jewel ever sold at auction, by Christie’s in Geneva for ca. US$ 57.9 million. The diamond was named for its previous owner Philip Oppenheimer.
The Oppenheimer Blue. Photo: Christie's.
The Pink Star diamond. Photo: Sotheby’s.
Other Noteworthy Diamonds
The Incomparable, discovered in 1984 in the Democratic Republic of Congo, was polished in the fourth-largest rough diamond ever found. Uncut, it weighed 890 carats.
The Incomparable diamond on a scale at SSEF. Photos: SSEF.
Although sometimes reported as weighing 407.78 carats, the SSEF tested the Incomparable in 2001 at 407.48 carats.
It is brownish yellow with an oblong, sometimes referred to as 'shield’ shape. Its cutting style is a mixed step-cut and modified brilliant-cut also referred to as 'fancy cut’.
「Introduction to Diamonds」
How Does a Diamond Form?
Diamond is valuable as a gemstone (and is also somewhat of a geologist’s best friend) because it provides a rare opportunity to study processes that occur deep in the Earth. Most diamonds form in the so-called lithospheric mantle within a fairly narrow depth range of 140 km to 200 km. At these great depths, subjected to high pressure and temperature, carbon crystallises into diamond. It is thought that the carbon originally came from sediments that were subducted from the Earth’s surface into the mantle region.
Interestingly, diamonds from deeper sources (called super-deep diamonds), some in excess of 600 km, have been reported as well, and continue to fascinate geologists.
Diamonds migrate to the Earth’s surface by very fast-moving kimberlite (a type of igneous rock) magma. Kimberlite is not the mother rock of diamond per se, but the rock which transports it to or near the surface of the earth.
Many diamonds are also very old in age. Diamonds from the Ekati mine in Canada were dated as forming some 3.5 billion years ago, as were diamonds from Siberia and Zimbabwe (as a reference, the Earth is 4.6 billion years old).
It may be that diamonds form over periods of time as short as days, weeks, months – to millions of years. As we don’t have access to the great depths in which they form, many questions remain about the origin and formation of diamonds.
Diamond Mining Today
Diamonds can be found in primary deposits (ie in the kimberlitic rock that brought them to the Earth’s surface) or in so-called secondary deposits. Secondary deposits occur when the primary rock has been eroded (by water or wind for example) away and the diamonds are either found in situ or in river beds.
Artisanal miners on the Sewa River in central Sierra Leone. They dive for diamonds that are found in the river sediments. This is a so-called secondary alluvial deposit. Photo: Laurent Cartier, SSEF.
Main Sources of Diamonds Today
「 Grading of Diamonds」
Diamonds are graded using the 4C grading system: carat, clarity, colour, and cut. Other factors are also analysed and described by a diamond grading lab. The International Standard Organisation has developed a diamond grading standard which covers all aspects of diamond grading (ISO Standard 24016). This ISO standard was developed with the assistance of many partners, including the SSEF and the CIBJO.
Carat: The weight is given in carats (ct). A carat equals 0.2 g. The minimum weight for a diamond to be graded on an SSEF Diamond Grading Report is 0.25 carats.
Colour: The colour is graded in comparison with an official CIBJO diamond master series. The SSEF is custodian of the official original C1 diamond master set.
Depth and table proportions: The depth and the table dimensions of a diamond are expressed as a percentage of its average diameter (for a round shape) or of its width (for other shapes).
Girdle: The girdle of a diamond is described by its thickness (Very thick, Thick, Medium, Thin, Very thin, Sharp-edged) and its state (bruted, faceted, polished).
Polish: The polish of a diamond describes the quality of its facets’ surfaces and is expressed by the terms: Excellent, Very good, Good, Medium, or Poor.
Symmetry: The symmetry of a diamond is the exactness of its shape and the arrangement of its facets. It is expressed by the terms: Excellent, Very good, Good, Medium, Poor.
Other Characteristics of Diamonds:
Shape and cut style: The shape of a diamond describes its outline (eg round, octagonal). The cut of a diamond is defined by its facet arrangement (eg brilliant, step cut).
Measurements: are expressed in millimetres and are reported as follows. For a round shape:'minimum diameter’–'maximum diameter’x'depth’. For other shapes: 'length’x'width’x'depth’.
「SSEF Expeditions to Diamond Mines」
「Synthetic Diamonds」
Since its beginnings in the 1950s, the production of synthetic diamonds has progressed substantially. Nowadays, all synthetic diamonds in the trade are created by two currently known methods. The terms 'synthetic diamond”, 'laboratory-created diamond’, and 'laboratory-grown diamond’ are synonymous.
The first, high-pressure, high-temperature (HPHT) synthesis, uses a press to recreate the conditions at which diamonds grow in nature. The second, Chemical Vapour Deposition (CVD), is a method to artificially grow diamonds in a vacuum chamber under low pressure, using a plasma.
While growing gem-quality diamond by CVD was once considered exceptionally difficult, polished stones of over 5 carats have been reported in recent years. For HPHT, much larger sizes have been reported.
HPHT presses capable of producing HPHT synthetic diamonds. Photo: Laurent Cartier, SSEF.
Unlike all imitation diamonds such as zirconia, synthetic moissanite, or strontium titanate, a synthetic diamond has similar physical and chemical properties to a natural diamond. A synthetic diamond is as hard, as chemically inert, and as highly refractive as a natural diamond. However, synthetic diamonds differ to natural diamonds in their formation (time, pressure, temperature, trace-element chemistry), and can thus be distinguished using advanced gemmological lab techniques.
Uses of Synthetic Diamonds
Synthetic diamonds were long manufactured for industrial purposes. This was traditionally for tools used for cutting, sawing, grinding, and polishing. In recent years, many optical and technological applications make use of some of the exceptional properties of synthetic diamond materials. They have a strong future in industrial applications such as optics and quantum computing.
Synthetic diamonds are also increasingly used for jewellery, as both quality and size have increased. They must be properly declared as synthetic when sold to consumers.
「Treatments of Diamonds」
A diamond may be treated to improve its colour or its clarity. It is mandatory to disclose any diamond treatment to a consumer and in the trade.
The colour of a diamond (or synthetic diamond) may be modified by treatments such as:
Coating, foiling and painting
Irradiation (usually combined with subsequent heating)
Bombardment with subatomic particles
High-pressure, high-temperature (HPHT)
These treatments will modify the colour of a diamond, either by producing a more attractive colour (eg pink, by irradiation treatment) or by reducing an undesired brownish colour hue, thus finally also resulting in a more attractive colour (eg colourless after HPHT treatment).
The determination of colour authenticity (natural or artificial colour) of a diamond is often challenging and thus performed only by a few laboratories worldwide as it requires sophisticated lab instrumentation and expertise.
The clarity of a diamond (or synthetic diamond) may be modified by treatments such as：
Filling of fissures with high-refractive glass (eg Lead glass)
Laser drilling to reduce the visibility of inclusions
Whereas the introduction of a high-refractive glass can be very effective in hiding large fissures in a fractured diamond, laser drilling is rather less efficient, as it only reduces the visibility of larger inclusions but creates new laser drill channels within the diamond. Both treatments are readily detected by a trained gemmologist.
「Introduction to Diamonds」
Every month, the watch and jewellery industries consume millions of colourless, small, round polished diamonds known as melee. Melee diamonds are diamonds that weigh less than 0.25 carats or approx 4.1 mm in diameter. Melee diamonds can be as small as half a millimetre (0.5 mm).
Although these diamonds are not individually graded, they can be tested for quality control (to ensure that they conform to a certain range of size, colour, and clarity). Quality control of small diamonds is complex and requires considerable experience.
In 2013, the SSEF and SATT Gems developed the first-ever instrument to authenticate round polished colourless diamonds ranging from 1.0 mm to 3.8 mm in diameter in an automated manner: ASDI (Automated Spectral Diamond Inspection).
In 2018, the ASDI machine was updated to enable the authentication of melees as small as 0.85 mm. Its average sorting speed now exceeds 6,000 stones per hour. The machine automatically separates natural colourless diamonds from colourless diamond imitations, colourless HPHT-treated diamonds, and both HPHT and CVD grown colourless synthetic diamonds.
「Introduction to Diamonds」
Scientific testing of diamonds using advanced spectroscopic techniques is vital in answering important questions such as
What type is the diamond (I or II and subtypes)?
Is it a natural or synthetic diamond?
If synthetic, by which process was it produced (CVD or HPHT)?
Has it been treated or not, and consequently, is the colour natural or not?
Determining the diamond type
Using Fourier-transform infrared spectroscopy (FTIR), we can classify diamonds in different types.
Type I: diamonds containing traces of nitrogen (N) as chemical impurities.
Type II: diamonds free of (FTIR-detectable) nitrogen, but may contain traces of boron (B).
Furthermore, we can distinguish several subtypes, based on the way how these nitrogen or boron impurities are present within the crystal structure of the diamond.
Pure diamonds without nitrogen and boron are Type IIa. They represent only about 0.1% of natural diamonds. However, as many of them are quite large and absolutely colourless, they are especially sought after in the gem trade. Some legendary and historic diamonds are of this Type IIa, such as the Koh-i-Noor and the Beau Sancy diamonds (see Famous Diamonds chapter).
Diamonds with traces of boron are Type IIb. Their colour is blue (to greyish blue) due to the presence of boron. Famous examples are the Hope, the Wittelsbach and the Oppenheimer Blue.
Nitrogen as a trace impurity results in a yellowish diamond colour. The yellow colour saturation is related to the amount and aggregation state of the nitrogen atoms in the diamond structure. More than 97% of all diamonds used in jewellery are of Type IaAB (a sub-type of Type Ia); thus, they contain traces of nitrogen aggregated as so-called A- and B-aggregates.
Crystal lattice illustrations for different types of diamonds. Yellow represents nitrogen atoms, blue, boron; and light green, a vacancy. Illustrations: Laura Speich, SSEF.
It is very important for gemmologists to determine the type of a diamond, as it provides crucial information when assessing whether the diamond is natural or synthetic, and to determine whether and how it has been treated.
Differentiation between natural and synthetic diamonds:
A separation of natural from synthetic (lab-grown) diamonds relies on a combination of classic gemmological observations and sophisticated analytical methods, such as FTIR, and low-temperature photoluminescence (PL).
Another option is to visualise the internal growth structure of a diamond using the DiamondViewTM instrument. This is based on the fact, the growth of synthetic diamonds (specifically HPHT synthetic) is characteristically different from the growth pattern of natural diamonds.
Detection of diamond treatments:
Diamonds which have been treated to improve their clarity are rather easy to recognise using classic gemmological testing. Usually, a meticulous microscopic investigation will reveal its tell-tale features (see the photo in the chapter on diamond treatments).
However, it is generally much more challenging to detect treatments that modify the colour of a diamond. This requires a combination of sophisticated analytical methods such as FTIR, and more specific absorption and photoluminescence spectroscopy. For these methods, the diamond is placed in liquid nitrogen (-196°C), where the quality of the spectra is distinctly improved by the extreme temperature.
The absorption spectrum of an HPHT treated diamond of Type Ia of greenish-yellow colour, which was taken at low temperature (in blue) by cooling with liquid nitrogen, is much more detailed as when analysed at room temperature (in red). Spectra: SSEF.
To fully understand the structural (vibrational) and photoluminescent characteristics of a diamond, it is helpful to analyse the diamond using lasers with different emission wavelengths. Collecting multiple spectra of a diamond can provide characteristic information useful in determining whether the diamond is natural or synthetic and whether it has been subject to a treatment (eg HPHT).
Testing and Reports at SSEF
A sample SSEF diamond grading report.