Natural or Laboratory-Grown: Diamond Inclusions That Can Be Confusing
By Nicholas DelRe
Fig 1. This HPHT-grown diamond had an inclusion that appeared to be a laser-drill hole, which is an uncommon treatment for HPHT diamonds and could cause a gemologist to make an incorrect growth origin determination. Image @ GSI
Background
Typical inclusions for HPHT laboratory-grown diamonds include flux remnants that come from the metallic catalysts such as iron, cobalt, and nickel that are used in its growth process. These flux remnants can occur in the form of pinpoints, clouds, and metallic “blobs” that can sometimes resemble included crystals found in natural diamonds. Similarly, they can occur in a variety of sizes, shapes, and locations within the body of the stone. (Fig. 2) HPHT-grown diamonds can also have fractures and fissures that may occur, which can be similar in appearance to natural diamond surface-reaching breaks and are rarely diagnostic.
Fig 2. Left: (a) A rough HPHT-grown diamond crystal with the classic cuboctohedral morphology. Note the rod-shaped metallic flux growth remnant in the center of the crystal. Right: (b) A magnfied view of a metallic flux inclusion in an HPHT-grown diamond that could be mistaken for a natural crystal inclusion. Image @ GSI
As the HPHT-grown diamond crystals are cut and polished, they undergo a final boiling in a powerful acid bath as a final clean-up of any micro residues such as graphite and that may have been left on the surface or from within existing surface-reaching fissures.
Observation
Upon initial gemological observation with a binocular darkfield microscope, the inclusion in question had the look of a laser drill hole. (Fig. 3)
Typically, the most likely candidate(s) for laser drilling are natural diamonds that have dark inclusions. The goal is to lighten the dark inclusion by introducing an acid through the drill hole acting as a channel that establishes contact between the surface of the diamond and the inclusion.
Laser drilling in HPHT-grown diamonds would be unlikely as a treatment because the metallic inclusions found in HPHT laboratory-grown diamonds are usually not as dark and tend to have more of a grayish metallic appearance. As such, they may not have the same relief or visibility as a darker sulfide or graphite inclusion which would benefit from the acid bleaching.
Fig 3: Left: ( a) The sample inclusion that appeared to be a laser drill hole. Right: (b) A laser drill hole in a natural diamond that reaches an inclusion for bleaching to improve face-up appearance. Note the resulting stress fracture and floret pattern due to the acid etching. Image @ GSI
Analysis
The finding of laser drilling in a laboratory-grown diamond is unusual but should not be ruled out. Although it has a strong resemblance to a laser drill hole (Fig. 3) or even a needle or etch channel in a natural diamond, other views of this elongated channel like inclusion supported an alternative explanation.
Fig. 4: Left: (a) The face-up view in reflected light shows a tension fracture and a hole that reaches the surface. Right: (b) The texture of the void appears smooth and consistent, unlike the irregular texture of etch channels that appear in natural diamond. Image @ GSI
Taking the diamond cutting process into consideration helps better understand what may have caused the feature. It is likely that in this case, there was previously an elongated metallic inclusion where both ends happened to be cut and exposed on the crown and pavilion surface of the polished laboratory-grown diamond. This exposure to the surface enabled the acids used in the cleaning process to dissolve the flux mass. The result was a hollow channel with a smooth surface, reminiscent of the smoother texture of a laser drill hole. In this case, the rounded channel did not lead to an inclusion that had been bleached so a laser drill hole was excluded as the cause of the feature.
Another consideration was the comparison to etch channels found in natural diamonds. They are natural features that occur rarely and are the result of the dissolution of defects in the crystal structure by caustic fluids found deep in the earth’s surface where diamonds are formed. The high-temperature, high-pressure fluids etch through the diamond, changing path and direction as inhomogeneities in the crystal structure are encountered. This can result in channels that are singular or have unusual patterns.
Fig. 5: Top left (a) Face-up view of a network of dissolution etch channels in a diamond. Top right: (b) Close-up view of etch channels breaking the surface. Bottom left (c) Natural etch channels in natural diamond. Note the oxide staining from the fluids that caused the etching. Image @ GSI
Piecing the gemological evidence together, GSI gemologists concluded that the observed inclusion was a result of a metallic flux inclusion from HPHT growth being dissolved as part of the standard polishing process, as laser drill holes and etch channels did not fit the observations although there were striking visual similarities.
Conclusion
With more laboratory-grown diamonds in the marketplace, there will be a corresponding necessity to thoroughly screen and test for laboratory-grown diamonds, natural diamonds, and their simulants so that inclusions such as this one can facilitate an accurate determination and avoid misinterpretations of the growth method.
The challenges of examining diamonds under a microscope include reflection and refraction, and incorporating different viewing angles, backgrounds, magnifications, and lighting compound with inclusions that can overlap in appearance. It is also essential to note that the presence or absence of inclusions is not diagnostic in determining if a diamond is natural or laboratory grown.
Inclusions can play an important role in distinguishing natural from laboratory-grown diamonds, but it also requires experienced gemologists to make sound deductive decisions about how the inclusions tell the story of the growth method of the diamond. Most importantly, send the diamond to a knowledgeable gemological laboratory such as GSI when in doubt.