The Nature of the diamond
Natural diamond crystals formed in the earth millions of years ago, 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 extracted to recover the diamonds and the ore is broken down mechanically to release the crystals.
The amount of diamond in the kimberlite is very small, about one part per million. Miners must therefore process large quantities of ore to recover the diamonds. Natural diamonds develop under particular temperature and pressure conditions. Temperatures are higher than those used to grow synthetic diamonds. At high temperatures, diamonds grow like octahedral crystals, but in the lower temperatures of the laboratory they develop as octahedral and cubic crystals. The large ERA of natural diamonds means that nitrogen impurities in most diamonds had time to aggregate into pairs or clusters, making the vast majority – more than 95%.
Synthetic diamonds are grown in a very short time, from a few weeks to a month, obviously in 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 than natural diamond.
Scientists began cultivating synthetic diamonds in the mid-1950s as tiny crystals. Production of larger crystals suitable for jewelry use began in the mid-1990s and continues today with the participation of more companies in diamond growth. Synthetic diamonds are grown in different countries for jewelery and industrial uses, which may be the most important use for this material.
The traditional synthesis method, called high pressure high temperature growth (HPHT), involves the formation of diamonds from a molten metal alloy, such as iron (Fe), nickel (Ni) or cobalt (Co). The most recent method, known as chemical vapor deposition (CVD) or low-temperature and high-temperature growth (LPHT), involves the formation of diamonds from a gas in a vacuum chamber where particles react to create layers of carbon that gradually consolidate into a single stone. In both methods, a crystal or diamond plate is used as a seed to start growth.
HPHT diamond growth occurs in a small capsule inside an apparatus capable of generating very high pressures. Inside the capsule, the starting material of the diamond powder dissolves in the molten metal stream and then crystallizes on the seed to form the synthetic diamond crystal. The crystallization takes place over several weeks, or even a little over a month, to create one or more crystals.
Synthetic HPHT diamond crystals generally have cubic faces in addition to octahedral ones. Since the shapes of the natural synthetic diamond crystals and HPHT are different, their internal growth profiles also differ considerably. These growth models can be one of the most reliable ways to separate them. The resulting faceted synthetic gemstones often have visual characteristics such as color distribution, fluorescence zonation and graining patterns related to their cross-shaped growth structure, as well as the occasional presence of dark metal inclusions. In some cases, the material exhibits persistant phosphorescence after the extinction of the ultraviolet lamp.
These synthetic diamonds can be identified using laboratory techniques such as visible spectroscopy and photoluminescence.
Most crystals obtained with HPHT are yellow, orange-yellow or brownish-yellow. Almost all are type IIb, which is rare in natural diamonds. The creation of colorless HPHT synthetics has been a challenge as changes in growing conditions and equipment to exclude nitrogen are needed. In addition, the growth rates of high-grade colorless diamonds (type IIa or low type IIb) are lower than those of type Ib synthetic diamond, which requires longer growth times and better control of temperature and pressure. While it was traditionally difficult to cultivate high-quality colorless HPHT crystals, recent developments have produced enough crystals for faceted stones weighing more than 10 carats. Adding boron to the growth system causes blue crystals. Other colors, such as pink and red, can be produced by post-growth processes involving radiation and reheating, but they are less common.
CVD diamond growth takes place inside a vacuum chamber filled with a carbon-containing gas, such as methane. An energy source, such as a microwave ray, breaks down the gas molecules and the carbon atoms are drawn down by the flat plates of the diamond. The crystallization takes place over several weeks to create a number of crystals; the exact number depends on the size of the piece and the number of seeds. Tabular crystals often have a rough black graphite edge. They also often have a brown color that can be removed with the heat treatment before the facet. Like HPHT synthesis, CVD synthesis continues to improve and allows manufacturers to offer larger sizes and improved color and purity.
In recent years, a growing number of companies have started producing synthetic diamonds for jewelry use . Their clarity and color have been continuously improved, as well as the carat weight.
To identify all types of precious stones, an experienced gemologist uses different types of test instruments, including a refractometer, an ultraviolet fluorescent lamp, a binocular microscope, a polariscope and other test instruments. As the quality of synthetic diamonds improves, it is increasingly difficult to separate them from natural gems using standard equipment.
Although an experienced gemologist may not be able to recognize synthetic diamonds, several factors can be used to identify a Synthesis treatment as follows:
|HPHT sintetico||CVD sintetico|
|Distribuzione dei colori non uniforme||Distribuzione dei colori non uniforme|
|Diversi modelli di graining||Nessun modello di graining|
|Colori insoliti di fluorescenza||Colori insoliti di fluorescenza|
|Modelli di colore a fluorescenza||Modelli di colore a fluorescenza|
|Fosforescenza occasionale||Fosforescenza occasionale|
|Inclusioni di flusso metallico||Punti scuri occasionali|
|Nessun modello di deformazione||Modelli di ceppo a bande|
|Possibile iscrizione sulla cintura||Possibile iscrizione sulla cintura|
These are visual features of most synthetic diamonds. All faceted synthetic diamonds will not have all of these features. For example, a particular synthetic diamond may have no fluorescence. Therefore, it is important to base the synthetic diamond identification on as many diagnostic functions as possible.
Colored synthetic diamonds grown with HPHT often have an irregular color that can be seen with light transmitted under a microscope and, if necessary, immersing the cut stone in water or mineral oil to minimize reflections on the surface area. This chromatic zonation is due to the way in which impurities, such as nitrogen, are incorporated into the formed synthetic diamond crystal. Sometimes natural diamonds have a chromatic zonation, but not in the geometric pattern presented by synthetic HPHT diamonds.
In contrast, synthetic diamonds grown with CVD normally have a uniform color.
HPHT synthetic diamonds often have solid molten metal inclusions, which appear black and opaque in transmitted light, but have a metallic luster in reflected light. Since the fluid used for diamond growth generally contains elements such as iron, nickel and cobalt, synthetic diamonds with larger metal inclusions can be collected using a magnet.
Synthetic diamonds grown with CVD are formed differently and dont have metal inclusions.
Some natural diamonds contain dark inclusions of graphite or another mineral, but these inclusions dont have a metallic luster. When examined between two polarizing filters oriented at a 90-degree angle, a natural diamond often has a dotted pattern or a light mosaic of interferential or “stress, sforzo” colors. These interference colors result from the fact that the diamond is subjected to stresses when it is deep in the earth or when it erupts explosively on the surface of the earth. In contrast, synthetic diamonds grow in a nearly uniform pressurized environment where they are not subject to stress. Therefore, if they are examined in the same way, they do not exhibit any deformation pattern or a low band deformation pattern. In addition, the fluorescence of synthetic diamonds is 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 have a cross-shaped fluorescence pattern on the crown or cut stone pavilion. Synthetic diamonds grown with CVD may have a ribbed pattern through the facets of the pavilion. Typical fluorescence colors are green, yellow-green, yellow, orange or red. When the ultraviolet lamp is extinguished, the synthetic diamond may exhibit a persistant phosphorescence for a minute or more.
The real identification challenge for the jewelery trade is the testing of very small diamonds sold in packages of several hundred to several thousand and which may include both natural and synthetic diamonds. To help the jewelery industry solve this problem, there are now automated tools created and developed to test even very small diamonds, called DiamondViews. (Synthetic diamond detector).
In summary, synthetic diamonds are now available in increasing quantities for jewelry use; we know that they can be identified with a DiamondView that checks samples of all sizes and colors using the three fundamental controls of photoluminescence, ultraviolet illumination and optical absorption. The detectable characteristics may vary under heat treatment (low or high pressure); these changes are identifiable and can allow unequivocal detection of synthetic diamonds grown with CVD or HTHP.
Our company has long introduced synthetic material detection technology using a SYNTHETIC DIAMOND DETECTOR, which is not a type separator, but identifies synthetic diamonds grown in the laboratory with CVD and HPHT methods and effectively features with all existing sizes, shapes and colors of diamonds.
Our goal is to maintain the trust you place in us and to preserve the natural diamond trade.