The dream of making a diamond from simple carbon has always been a captivating idea for scientists and visionaries. However, only recently, with the advent of new techniques, we were able to produce gem-quality lab-grown diamonds large enough to be used in jewellery. Although man-made diamonds are claimed to be identical to natural ones, some features set them apart. Let’s see how to identify synthetic diamonds.
What Are High Pressure High Temperature (HPHT) Diamonds?
Natural diamond crystals form very deep underground where conditions of extremely high pressure and high temperature are naturally found. That is why the first and the oldest technique to grow synthetic diamonds mimicked nature with high pressure and high temperature (HPHT).
There are three techniques to grow HPHT diamonds: the belt press, the cubic press and the split-sphere press. The goal of each is to create an environment of high pressure and high temperature where diamond growth can occur (pressure over 1.5 million pounds per square inch and temperature above 1400°C). Each process starts with a seed of a diamond material placed in carbon and put under extremely high pressure and temperature. In these conditions, carbon melts and forms into a diamond around the seed.
The first reproducible synthetic diamond was created using the belt press by General Electric on December 16, 1954. The diamond was tiny and heavily included, so it was not suitable for jewellery manufacturing. It is worth mentioning that even today most of the synthetic diamonds produced are not of gem quality. They are created for industrial purposes where tools and equipment of excellent properties (hardness, thermal conductivity, etc.) are used.
With the success of the belt press, HPHT technology has been advancing for years, and modern cubic and split-sphere presses have been introduced. These were more efficient and allowed to produce larger gem-quality diamonds. However, the HPHT diamond growing process is very expensive and produces diamonds with mainly yellowish and brownish tints.
What Are Chemical Vapour Deposition (CVD) Diamonds?
Chemical Vapour Deposition (CVD) is a more recent diamond growing technique during which a gemstone is grown from a hydrocarbon gas mixture.
During the CVD process, a thin wafer of a diamond crystal is placed in a sealed chamber and heated to 800°C. Then the chamber is filled with a carbon-rich gas, such as methane, and gets ionized into plasma using lasers, microwaves or others. The ionization breaks the molecular bonds in the gas, and pure carbon gets deposited on the wafer of a diamond crystal (usually HPHT synthetic), growing a gemstone atom by atom, layer by layer.
The CVD diamond growing process has a couple of advantages over the HPHT process. High pressure is not needed, and temperature, while high, does not need to be above 1400°C, meaning this process is much less costly. Finally, the CVD method can produce colourless gems.
How to Identify Synthetic Diamonds
Although synthetic diamonds have the same crystal structure and chemical composition as natural ones, the diamond growing process leaves unique features in the finished products. These features help to tell synthetic diamonds apart from natural ones and even allow to identify which method has been used to grow the diamond.
The biggest concern about synthetic diamonds is the non-disclosure of their origin and being sold as natural ones. The main thing to realize is that most synthetic diamonds are responsibly disclosed to be lab-created. There are many different industry terms such as synthetic, man-made, cultured or created diamonds, and they all mean the same thing – the diamond is not natural.
The majority of synthetic diamond manufacturers are serious organizations that are upfront about their products. Some companies even laser inscribe their diamonds’ girdles to mark them as lab-grown. Moreover, lab-grown diamond creators never miss the moment to highlight the ethical source of their product.
Type IIa Test
One of the best indicators of whether a diamond is natural or synthetic is the so-called Type IIa test. Type IIa is a gemological variant of a diamond that tells us how pure is carbon. Type IIa diamonds are almost free of elements like nitrogen, which makes them colourless.
Less than 2% of all mined diamonds are Type IIa, while most gem-quality synthetic diamonds are Type IIa. So this is a pretty good indicator that the gem is lab-grown. However, it is important to realize that there are still 2% of natural diamonds that are Type IIa, so this test is not 100% conclusive.
Colour zoning is an important identifying marker. HPHT coloured diamonds often display geometric colour zoning due to the concentration of nitrogen. Natural diamonds can also display colour zoning, but their pattern is never geometric. CVD diamonds, on the other hand, usually have even colouration.
HPHT synthetic diamonds often contain metal inclusions such as iron, nickel and cobalt, as these metals are used during the growth process, and sometimes they enter the diamond crystal.
CVD diamonds never contain metal inclusions, but they often come with graphite or other mineral inclusions, which are a result of their growth process.
When examined between two polarized filters, mined diamonds often show a bright mosaic or crosshatched pattern, also known as “strain”. CVD diamonds display banded strain patterns, a feature that helps to tell CVD from HPHT diamonds. In contrast, HPHT synthetics display either no strain pattern or a very weak banded pattern.
Synthetic diamonds exhibit strong and unusual fluorescence. Natural diamonds exhibit the strongest fluorescence under long-wave UV, while synthetic diamonds show the reverse reaction – they exhibit the strongest reaction under shortwave.
Although natural diamonds can fluoresce almost any colour, the most common colours seen are blue, yellow and white. Synthetic diamonds exhibit green, yellow-green, yellow, orange and vivid red. Lab-grown diamonds can also exhibit phosphorescence when they glow for a minute or more, even after the UV lamp is turned off.
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