The Nature of the Diamond
Natural diamond crystals formed millions of years ago in the earth, at a depth of about 160 km and were brought to the surface much later by volcanic explosions. These eruptions formed narrow vertical pipes of an igneous rock called kimberlite. The kimberlite tubes are mined to recover the diamonds and the ore is mechanically broken down to release the crystals.
The amount of diamond in kimberlite is very low, about one part per million, so miners have to extract large quantities of ore to make diamonds. Natural diamonds grow under particular conditions of pressure and temperature. The latter much higher than that used to cultivate synthetic diamonds. So while at high temperatures, natural diamonds grow as octahedral crystals, synthetic diamonds obtained at lower temperatures grow as crystals with octahedral and cubic faces.
Synthetic diamonds are grown in a very short time, from several weeks to just over a month, obviously under conditions other than the formation of natural diamonds in the depths of the earth. Due to the very short growth period, the shape of a synthetic diamond crystal is very different from that of a natural diamond.
DIAMONDS OF SYNTHESIS
In the mid-1950s, the first scientists developed the rudiments of the cultivation of synthetic diamonds, but obtained only tiny crystals.
The production of larger crystals, usable in the production of jewelry, began in the mid-90s and is still evolving today. Synthetic diamonds are grown in various countries around the world and their main use, as well as in jewelry, is for industrial applications.
The traditional synthesis method, called “High pressure-High temperature” (HPHT) growth, synthesizes diamonds from a molten metal alloy, such as iron (Fe), nickel (Ni) or cobalt (Co) . The newer method, called “Chemical Vapor Deposition” (CVD) or low pressure and high temperature growth (LPHT), involves the formation of diamonds from a gas in a vacuum chamber where the particles react between of them creating layers of carbon that gradually consolidate into a single stone. In both methods, a crystal or a diamond plate is used as a seed to start the growth
HPHT SYNTHESIS
La crescita del diamante HPHT avviene in una piccola capsula all’interno di un apparato in grado di generare pressioni molto elevate. All’interno della capsula, il materiale di partenza della polvere di diamante si dissolve nel flusso di metallo fuso e quindi cristallizza sul seme per formare il cristallo di diamante sintetico. La cristallizzazione avviene nell’arco di diverse settimane fino a poco più di un mese per creare uno o pochi cristalli.
I cristalli di diamanti sintetici HPHT in genere mostrano facce cubiche oltre a quelle ottaedriche. Poiché le forme dei cristalli di diamanti sintetici naturali e HPHT sono diverse, anche i loro modelli di crescita interna differiscono notevolmente. Questi modelli di crescita possono essere tra i modi più affidabili per separarli.
Le gemme sintetiche sfaccettate risultanti mostrano spesso caratteristiche visive come la distribuzione dei colori, la zonazione della fluorescenza e i modelli di graining correlati alla loro struttura a settore di crescita a forma di croce, nonché la presenza di occasionali inclusioni di metallo scuro.
In alcuni casi il materiale presenta una fosforescenza persistente dopo lo spegnimento della lampada a ultravioletti. Questi diamanti sintetici possono essere identificati in modo positivo utilizzando tecniche di laboratorio come la spettroscopia visibile e la fotoluminescenza.
La maggior parte dei cristalli cresciuti con HPHT sono gialli, giallo arancio o giallo brunastro. Quasi tutti sono di tipo IIb, che è raro nei diamanti naturali. La creazione di sintetici HPHT incolori è stata una sfida, poiché sono necessarie modifiche alle condizioni di crescita e alle attrezzature per escludere l’azoto. Inoltre, i tassi di crescita per diamanti incolore di elevata purezza (tipo IIa o tipo IIb debole) sono inferiori a quelli del diamante sintetico tipo Ib, che richiede tempi di crescita più lunghi e un maggiore controllo sulle condizioni di temperatura e pressione. Mentre tradizionalmente è stato difficile coltivare cristalli HPHT incolori di alta qualità, recenti sviluppi hanno prodotto cristalli sufficienti per le pietre sfaccettate superiori a 10 carati di peso . L’aggiunta di boro nel sistema di crescita provoca cristalli blu. Altri colori, come il rosa e il rosso, possono essere prodotti da processi di trattamento post-crescita che coinvolgono radiazioni e riscaldamento, ma sono meno comuni.
The growth of the HPHT diamond occurs in a small capsule inside an apparatus capable of generating very high pressures. Inside the capsule, the starting material, diamond powder, dissolves in the flow of molten metal and then crystallizes on the seed to form the synthetic diamond crystal. The crystallization of one or a few crystals takes place over a period of a few weeks up to just over a month.HPHT synthetic diamond crystals typically show cubic as well as octahedral faces. Since the crystal shapes of natural and HPHT synthetic diamonds are different, their internal growth patterns also differ greatly. These growth patterns may be among the most reliable ways to tell them apart.The resulting faceted synthetic gemstones often exhibit visual characteristics such as color distribution, fluorescence zoning, and graining patterns related to their cross-shaped growth sector structure, as well as the presence of occasional dark metal inclusions.In some cases the material exhibits a persistent phosphorescence after the ultraviolet lamp is turned off. These synthetic diamonds can be positively identified using laboratory techniques such as visible spectroscopy and photoluminescence.
Most of the crystals grown with HPHT are yellow, orange yellow, or brownish yellow. Almost all of them are type IIb, which is rare in natural diamonds. Creating colorless HPHT synthetics has been a challenge, as changes to growing conditions and equipment are needed to exclude nitrogen. Furthermore, the growth rates for high purity colorless diamonds (type IIa or weak type IIb) are lower than those of type Ib synthetic diamond, which requires longer growth times and greater control over temperature and pressure conditions. While traditionally it has been difficult to grow high quality colorless HPHT crystals, recent developments have yielded enough crystals for faceted stones over 10 carats in weight. The addition of boron in the growth system causes blue crystals. Other colors, such as pink and red, can be produced by post-growth treatment processes involving radiation and heating, but are less common.
CVD SYNTHESIS
The growth of the CVD diamond occurs inside a vacuum chamber filled with a gas containing carbon, such as methane. An energy source, such as a microwave beam, breaks the gas molecules and the carbon atoms are drawn downward by the flat diamond plates. Crystallization takes place over a period of several weeks to create a certain number of crystals; the exact number depends on the size of the chamber and the number of seeds. Tabular crystals often have a rough edge of black graphite. They also often show a brown color that can be removed by heat treatment before faceting. Like HPHT synthesis, CVD synthesis continues to improve and allows manufacturers to offer larger sizes and improved color and purity.
IDENTIFICATION
In recent years, an increasing number of companies have started producing synthetic diamonds for jewelry production. There have been continuous improvements in their quality with regards to clarity, color and considerable increases in carat weight.
To identify the origin of the gem material, an expert gemologist uses different types of tools, including the refractometer, the ultraviolet fluorescence lamp, the binocular microscope, the polariscope and other test tools. As the quality of synthetic diamonds improves more and more, it is increasingly difficult to distinguish them from natural gems using standard equipment.
Therefore, even if an expert gemologist should not be able to recognize synthetic diamonds, there are several essential factors that identify a Synthesis treatment as shown below.
These are visual characteristics of most synthetic diamonds. Not all faceted synthetic diamonds will exhibit all of these characteristics. For example, a particular synthetic diamond may not show any fluorescence. Therefore, it is important to base the identification of the synthetic diamond on as many diagnostic characteristics as possible.
Synthetic colored diamonds grown with HPHT often exhibit an uneven coloring that can be seen with transmitted light using the microscope and, if necessary, soaking the cut stone in water or mineral oil to minimize surface reflections. This chromatic zoning is due to the way in which impurities, such as nitrogen, are incorporated into the synthetic diamond crystal that is formed. Occasionally, natural diamonds exhibit chromatic zoning, but not in the geometric pattern shown by HPHT synthetic diamonds.
Conversely, synthetic diamonds grown with CVD normally exhibit a uniform color.
HPHT synthetic diamonds often show inclusions of solid molten metal, which appear black and dull in transmitted light but have a metallic luster in reflected light. Since the flux alloy used for diamond growth usually contains elements such as iron, nickel and cobalt, synthetic diamonds with larger metal inclusions can be picked up with a magnet.
Synthetic diamonds grown with CVD are formed in a different way and have no metallic inclusions.
| HPhphtHT | CVD sintetico |
|---|---|
| Uneven color distribution | Uneven color distribution |
| Different models of graining | No graining pattern |
| Unusual colors of fluorescence | Unusual colors of fluorescence |
| Fluorescence color models | Fluorescence color models |
| Occasional phosphorescence | Occasional phosphorescence |
| Metallic flux inclusions | Occasional dark spots |
| No deformation model | Banded strain patterns |
| Possible inscription on the belt | Possible inscription on the belt |
Some natural diamonds contain dark inclusions of graphite or some other mineral, but these inclusions do not have a metallic luster. When examined between two polarizing filters oriented at an angle of 90 degrees to each other, a natural diamond often exhibits a dotted pattern or luminous mosaic of interference or “effort” colors. These interference colors result from the diamond being subjected to stresses while it was deep in the earth or during its explosive eruption on the earth’s surface. In contrast, synthetic diamonds grow in an almost uniform pressurized environment where they are not subjected to stress, so when examined in the same way, they show no deformation pattern or a weak band deformation pattern. The fluorescence of synthetic diamonds is also often very useful for identification, it is often stronger under a short wave than a long wave ultraviolet lamp and can show a distinctive pattern. Synthetic diamonds grown with HPHT tend to exhibit a cross-shaped fluorescence pattern on the crown or pavilion of the cut stone. Synthetic diamonds grown with CVD can exhibit a streaked pattern when viewed across the pavilion facets. Typical fluorescence colors are green, yellow-green, yellow, orange or red. When the ultraviolet lamp is turned off, the synthetic diamond may exhibit persistent phosphorescence for up to a minute or more.
The real identification challenge facing the jewelry trade is experimenting with very small diamonds that are sold in packages of several hundred to several thousand and which can include both natural and synthetic diamonds. To help the jewelry trade deal with this problem, there are now automated tools created and developed to test even very small diamonds called DiamondView
(Synthetic Diamond Detector).
In summary, synthetic diamonds are now being made available in increasing quantities for the use of jewelry; we know that they can be identified with a DiamondView that verifies samples of any size and color using the three basic checks which are photoluminescence, ultraviolet illumination and optical absorption. The detectable characteristics may vary under heat treatment (low or high pressure); these changes are identifiable and can provide unambiguous detection of synthetic diamonds grown with CVD or HTHP.