Understanding Lab Diamond Inclusions

The mineral known as Diamond is a crystalized form of the element Carbon. We tend to think of diamond as “pure” carbon, but at some level every diamond (even a flawless diamond) contains traces of other elements or minerals. Inclusions are observable internal characteristics of a diamond that can be seen under magnification. Some lab diamond inclusions are quite distinctive, while others look very similar to those seen in natural diamonds.

Natural diamonds that form over millions of years many miles beneath Earth’s crust can have a wide range of inclusion types including minerals and other gemstones such as sapphires, garnets, and even other small diamonds! Lab grown diamonds are created under highly controlled conditions and therefore do not show the same kind of diversity, but often contain some inclusions unique to synthetic diamonds. The type of inclusion observed in inspecting a diamond can then be diagnostic of its origin. But lab grown diamond inclusions can look very similar to those found in nature. And in the case of lab diamonds of VVS and IF clarity, there is little to nothing to observe. Therefore, inclusions are only one way to make a distinction between lab diamonds and natural diamonds, and in many cases should not be the only factor relied upon to make a conclusive identification.

Though we tend to think of synthetic materials produced in tightly controlled environments as being “perfect” replicas of natural materials, the process of creating lab diamonds is still subject to many variables which can introduce a variety of defects into the finished product. And the two distinct processes used to create diamonds (HPHT and CVD) are very different and introduce somewhat distinctive inclusions and submicroscopic defects. It seems likely that eventually it will be possible to produce diamonds of almost perfect purity, which is particularly important to certain industrial uses of lab grown diamonds. Already today the technology is producing volumes of diamonds that grade in the colorless (DEF) and VVS range, as well as IF diamonds. But even IF clarity may have inclusions visible at greater than 10X magnification, and certainly contain atomic level defects such as trace elements and aberrations in the carbon lattice.

The inclusions in lab grown diamonds are often referred to on laboratory reports as “growth remnants”. This is a general term for things like metallic flux particles in different forms, non-diamond carbon inclusions, and other characteristics specific to the process of synthesizing diamond.

Metals such as nickel and iron are used in the synthesis of diamond to act as catalysts or ‘flux’ to enable carbon atoms to separate and migrate to the seed plate. In some instances the metals do not fully melt in the chamber and pieces get trapped within the layers of diamond while it forms, leaving a metallic inclusion that may be amorphous or needle-like. Flux may also be trapped in its molten form creating different inclusion forms.

Non-diamond carbon inclusions such as graphite may also be trapped in the carbon lattice, giving rise to other forms such as very distinctive comet-like inclusions. Graphitic inclusions can also be in pinpoints and if numerous enough can impart a gray body color to the diamond.

In HPHT grown diamonds the growth chamber includes metals such as iron, nickel, cobalt (and possibly others) that are super heated under tremendous pressure and act as catalyst for diamond formation. Sometimes these metals do not fully dissolve and fragments of them are trapped within the diamond as it forms. These metallic inclusions take various forms including amorphous shapes and rod-like shapes. In reflected light they have metallic luster and in transmitted light they are opaque. Interestingly, some HPHT grown diamonds have enough metallic flux inclusions to be magnetic!

Somewhat less commonly seen, branchlike or “dendritic” inclusions can resemble trees or bushes when dark, or snow-covered branches when light. They occur when flux becomes trapped by the growing crystal while still in its molten state.

The CVD method does not use molten metal as a catalyst. The growth chamber contains carbon rich gasses such as methane and uses microwave energy to heat them into a plasma that separates the carbon atoms which precipitate onto a diamond seed plate. Some of that carbon can get trapped during growth in non-diamond form, such as graphite. Graphitic inclusions in CVD diamonds can sometimes look like crystals or the “carbon spots” seen in natural diamonds, but they are usually in a distinct plane in the crystal due to how CVD diamonds form a layer at a time. They are different from metallic inclusions in HPHT in that they are non-reflective.

According to GSI, “CVD-grown synthetic diamonds do not have metallic inclusions. Rather, they often contain dark graphite or other mineral inclusions that are a result of their unique growth process. Graphite inclusions appear different from metallic, in that they do not have a metallic luster. Inclusions in CVD diamonds occur as small dark particles or crystals that are generally non-diamond carbon.”

A distinctive inclusion found in CVD diamonds takes on a comet-like form. These unique ‘cometic’ inclusions appear in a grow growth plane as a result of stops and starts in the process. Triangular inclusions of graphitic inclusions are another unique feature in CVD diamonds.

Since lab diamonds have the same physical properties as natural diamonds they have certain features that have nearly exact counterparts in nature such as feathers (small fractures), bruises, abrasions, naturals, chips, scratches, graining and polish lines. In some cases they have inclusions that appear to be very ‘natural’ looking, so it can be tricky for a gemologist to make a definitive identification based on inclusions alone. This is especially true with diamonds in the VS1 and above grade range. For example tiny graphitic inclusions in a CVD diamond can look just like pinpoint crystals in a natural diamond.

The images below are from a diamond that was submitted to GIA that had many inclusions (graded Si2) but were mixed randomly, similar to how inclusions in natural diamonds are distributed. It also had what appeared to be etching, an inclusion sometimes seen in natural diamonds. But advanced testing revealed this stone to be a CVD grown diamond.

GIA researchers described the inclusions in this CVD as being “scattered in a random way showing no uniformity in size, shape, color, or placement. They were whitish or black, pinpoint-sized and larger structures. Some were flat and pointed, while others were more rounded in shape. Furthermore, some were grouped together, similar to cloud-like inclusions, while others were freestanding, mimicking the inclusion distribution in natural diamond.”

Certain lab grown diamond inclusions can easily be mistaken for natural inclusions. Planar clouds, and internal graining can look very much like similar features like clouds, twinning wisps and graining found in natural diamonds. So while inclusions can help us distinguish natural from lab grown diamonds, other tests are often necessary to make a positive identification.

While a small flux inclusion or growth remnant in a VS or VVS diamond has negligible impact on the light performance or beauty of a lab diamond, other defects in the growth process can have significant negative effects. And they often are not contained on reports from gemological laboratories.

Some features fall between inclusions that are easy to see with 10x magnification (which is the standard for diamond clarity grading) and the submicroscopic level, and thus may not be detailed in any direct way on a laboratory grading report. Strain and striation of the carbon lattice are important examples.

Striation is similar to grain lines in a natural diamond. It can also appear similar to twinning wisps, a common feature in natural diamonds. Strain in the crystal lattice is another factor common in lab grown diamonds, especially CVD grown, and relatively rare in nature. Both strain and striation can have a deleterious effect on a diamond’s transparency. If severe enough the diamond will take on a hazy look as light ray are scattered as they are propagated internally before being returned to the eye. This can be a rather subtle effect that takes a trained eye to assess. But if an appreciable transparency deficit is present a diamond will never achieve its full light potential, no matter how well cut it is.

While striation is something that can potentially be seen in a microscope and captured in magnified images, strain can only be directly observed in a polariscope or a microscope equipped with polarizing filters. However, when prominent enough both can be observed in the effect they have on the transparency of the diamond. Strain can manifest in what some researchers describe as a roiled appearance sometimes referred to as the ‘scotch and water’ effect. While not obviously hazy, the diamond can look a little blurry as though the reflections of the facets are somewhat out of focus.

Another issue that is somewhat specific to lab diamonds surrounds certain effects caused by trace elements present in the growth environment. While not falling into the category of inclusions per se, they are atomic level defects that can cause visible effects. The most well known in this category is an effect known as ‘blue nuance’ which is seen in some lab grown diamonds, particularly those grown by the HPHT method. This effect is caused by traces of the element Boron which is commonly used in this method. Boron is the same element that gives natural fancy blue diamonds their color and which is extremely rare in nature. Blue nuance, as the name implies, is a very subtle blue hue that can be seen in some lab diamonds, even in those getting top color grades from the laboratories. While blue is arguably an attractive color that some may find very appealing, because it is an indicator of synthetic origin it is seen as a negative trait by the lab grown market.

Other color tinges that may occur in lab grown diamonds are more common in natural and are less attractive, such as brown and gray. CVD grown diamonds are often treated to remove unwanted color. Such treatment is not always successful or may have unintended consequences. Many shoppers therefore look for “As Grown” diamonds without any evidence of post growth treatments.

Diamonds, both natural and lab grown, almost always have inclusions, trace elements, or growth defects that are both distinctive to the growth method and similar in appearance. Metal flux inclusions and some non-diamond carbon inclusions can be diagnostic for lab grown diamonds. Mineral inclusions such as garnets, sapphires and other minerals can be diagnostic for natural diamonds. But many inclusions in both lab grown and natural diamonds have a very similar appearance, so gemologists need to be very careful in making identification solely on the basis of inclusions. This is particularly true of very clean diamonds with few inclusions that are very small and difficult to resolve, even under the microscope. And since lab diamonds have the same physical properties as natural diamond, blemishes such as chips, scratches and polishing lines can be present in both. Other tests are usually necessary for conclusive identification.