What are CVD synthetic diamonds?

As such there are two methods of manufacturing synthetic diamonds high pressure high temperature (HPHT) which is growth from molten metallic solvent catalyst like iron-nickel and the other chemical vapour deposition (CVD) which is growth by converting methane gas into diamond.

  
HPHT method and CVD method for Diamond Synthesis

Chemical Vapor Deposition (CVD) is one of the techniques to produce coatings on substrates. Synthetic CVD diamonds are produced from the dissociation of carbon containing gas where by a coating of crystalline diamond is produced on a substrate.

Graphite can be converted to diamond

Ideally the graphite inside a pencil can be converted to a diamond by using heat and pressure. Under a pressure of some 70,000kg/cm2 and a temperature to the tune of 20000C graphite can be converted to a crystalline diamond. The hexagonal lattice structure of the graphite undergoes a 3 dimensional change as shown in the following figure.

The HPHT Method uses two types of apparatus

The BELT Apparatus is used to grow large synthetic diamonds. Graphite carbon is dissolved in the molten metal (usually iron-nickel flux)and at particular temperature and pressure the carbon precipitates as crystalline diamond. It takes few daysto grow diamonds of millimetersizes whereas to grow micro diamonds it requires only few hours. The BARS apparatus or split spheres is a pressure cell placed inside a cube shaped cavity which is enclosed in an octahedral shaped cavity formed by eight anvils. It gives perfect diamond crystals that are cubo-octahedral in shape.

Dissociation of methane gas

Chemical vapor deposition, CVD is the most recent method used for commercial production of synthetic diamonds.In the CVD process methane gas CH₄ is dissociated at low temperature and pressure. The carbon atoms are then deposited on a substrate. This is done four steps:

• 1. Pressure in the growth chamber is reduced to almost 1/10th of atmospheric pressure.
• 2. Methane gas (CH₄ gas) is injected and with microwave beam heated to 1000⁰C, plasma is formed CH₄ = C + 2H₂this type of reaction takes place.

• 3. Carbon atoms separate out and are slowly deposited on a substrate. Often the substrate is made of diamond wafer which acts like a seed crystal to start growth.

• 4. Thin wafers of crystalline carbon slowly grow into diamond bricks at the rate of 1cm thick in 20days (0.5 to 0.7mm per day)!

Synthetic CVD diamonds in form of slabs

Therefore it is possible to synthesize CVD diamond of good color and size as well as discs of large sizes.

Synthetic CVD diamond of F / VVS grade
Synthetic CVD diamond as a disc

In order to detect…

In order to detect we should have some idea about the different types among diamonds. Diamonds are classified into two Types depending upon nitrogen impurity present in them. Type I diamonds contain nitrogen impurities and Type II diamonds are those which lack detectable nitrogen. Further the Type I is further bifurcated into 2 types Type Ia and Type Ib depending upon the nitrogen atoms present in them.

Type II is bifurcated into two sub types: Type IIa (which has no nitrogen or boron impurities) and Type IIb (which has boron impurities).

According to GIA: more than 95 percent of natural diamonds turn out to be type Ia but that it is “difficult” to grow type Iadiamonds in a lab. CVD synthetics of any colour are Type IIa or Type IIb i.e. they do not have Nitrogen.

One can determine the diamond type by using Infrared spectroscopy

So, all those diamonds which show the peaks of the Type II category would be having the possibility of being synthetic diamonds.De Beers has come with two instruments DiamondSure and DiamondView which can be of great help for identifying synthetic diamonds.

Diamond View shows a typical red fluorescent image for synthetic CVD diamonds.The DiamondView gives high-energy UV radiation. If diamonds are exposed to it, then this allows for real-time imaging of fluorescence features in the diamonds, exposing their growth characteristic structure.

Tom Moses of GIA says that diamonds grown in a lab using the CVD or HPHT process exhibit a tell-tale growth patterns that are very characteristic of synthetic diamond. For example, of the 10 synthetics recently submitted to GIA Hong Kong all exhibited “typical CVD growth striations” when placed in the DiamondView, Moses said.“No matter what you do to treat or disguise synthetics, those two growth features, in HPHT and CVD, cannot be removed. It can’t be changed,” says Tom Moses. One can also try the D-Screen diamond testerfrom HRD Belgium or the Diamond Spotter from SSEF, Switzerland. Both only indicate the Type II diamond but do not confirm whetherthe sample is Synthetic CVDdiamond.

One Must Remember.

The CVD technology for growing large colourless diamond has improved. Now CVD Diamonds are larger and of higher quality and are more frequently seen in grading labs around the world. CVD grown Synthetic Diamond are regularly grown for Industrial purpose and the properties can be tailored according to the requirements of Sizes up to 120 mm Ø (12cm), with thickness 0.01 - 2 mm sometimes with Blue color which are boron doped and very transparent optical grade.

Conclusion

In this scenario we have to accept the fact that CVD grown Synthetic Diamond are going to increase!Multiple tests are needed to determine if a diamond is a natural or lab grown therefore a single inexpensive machine that can instantly identify mined diamonds from synthetics seems very unlikely. Presently methods to detect them (FTIR, UV-Vis, DiamondSure, DiamondView etc.) are only viable for certain sized diamonds. But for star and melee sizes it is simply uneconomical to use these instruments. Unless a new instrument is invented to identify CVD grown small-sized diamonds, over the years in future, small diamonds with small crystalline inclusions will be more valuable as they can be identified definitely as nature grown!

Dr. JayshreePanjikar
FGA (U.K.), DGemG (Germany), FFIG, FGS,
Certified Diamond Grader HRD (Belgium)
Graduate Pearl GIA (USA)
Email : jayshreepanjikar@gmail.com