Diamond Transparency – The Overlooked Foundation of Diamond Light Performance

​​While we tend to think of gem diamonds as completely transparent, that is not always the case. We’ve all seen diamonds that were so poor in quality that they did not sparkle. And there is a continuum of quality from very dull to exceptionally brilliant. The “flaws” in diamonds that we normally consider making a difference in diamond beauty are inclusions and blemishes; aspects that are part of any reputable diamond report. But there are other defects that impact a diamond’s transparency that can be much less obvious, but often have a significant impact on the appearance and performance of a diamond. And some are not always captured in the clarity grade but which interfere with the diamond’s ability to fully process light. More information on common inclusions that can cause a loss of diamond transparency can be found on our cloudy diamonds page.

In this article we will focus on transparency deficits that are not captured on a standard laboratory report as they may not be visible at 10X magnification, the level at which the clarity grade is rendered, yet may still manage to cause differing degrees of blurring of the virtual facets and overall haziness. A diamond with a transparency problem will never be capable of optimal light performance, no matter how precisely cut it is.

• Diamonds vary in transparency due to internal inclusions, external blemishes, structural defects in the crystal, and deficiencies in cut craftsmanship
• Transparency deficits can result in haziness and blurring of the virtual facets
• Some causes of transparency loss are at the atomic level and do not factor into the clarity grade, such as crystal strain and striation of the carbon lattice.
• Few gemological laboratories provide a direct assessment of diamond transparency. DCLA is an exception
• It can take a trained eye to assess transparency accurately
• A diamond with an appreciable transparency deficit will never be capable of optimal light performance, no matter how precise the cut quality is.
• The best way to observe haziness is to direct a bright light through the girdle of the diamond while viewing the stone through the table.

In natural diamonds the most common factors compromising transparency are inclusions, which are plotted or mentioned on a laboratory report, and to some extent external defects like surface graining or polish deficiencies. And even though transparency is not directly measured or graded on the report, the clarity grade in conjunction with the clarity features can give us some clues about the extent of any potential transparency issues. Indirect clues are usually not available in the case of lab grown diamonds which today tend to be in the very high clarity grades (VS1 and above). Yet they commonly have atomic-level defects which can impair transparency.

It is theoretically true that any inclusion in a diamond will block or scatter some light and prevent it from returning to the eye, thereby impacting light performance. But since diamonds are rarely, if ever perfect - even a flawless diamond is only deemed flawless at 10X - we will focus on transparency factors that impact light performance in an “appreciable” way.

The most common effect seen in lab diamonds is haziness or blurriness, usually due to atomic-level disruptions caused by crystal strain and/or striation of the carbon lattice. This is more often seen in diamonds grown by the CVD method, especially in larger sizes. Since the growth of the diamond in this method is not rigidly constrained by high pressure from every direction, distortions in the carbon lattice can more easily develop, as can stopping and restarting the process which is more common in CVD. If present to a sufficient degree strain and striation can impede the passage of light through the material degrading internal reflections. The result is a diminished quantity and quality of light return. Sometimes the diamond is obviously hazy, but in most cases it is subtle and may take a trained eye to assess accurately. In some cases images or videos may reveal striation from certain angles. Crystal Strain is best assessed under a microscope using polarized light.

Interestingly, because laboratory grown diamonds are grown quickly in highly controlled conditions, many of them, particularly those grown in a single process by HPHT without need for secondary treatment, often achieve transparency levels higher than most natural diamonds that develop over millions of years in the furnaces deep under the crust of the earth.

Striation of the carbon lattice is essentially the same thing as graining. You can think of it as similar to the grain in wood. As a tree grows slowly layer by layer environmental changes can result in changes to the color and texture of the wood. This can be seen in the patterns of a cut piece of wood and which may be positive in terms of beauty. In diamond growth the grain is rarely an overt visual property. Rather, it is an intrinsic property of the crystal that can potentially impede light rays from passing freely through it. This can impact the quantity and quality of light that is returned to the eye.

For optimal light performance a diamond must be proportioned correctly so that all the facets function in concert as mirrors internally reflecting light gathered by the crown, and providing the proper exit point back through the crown, allowing the light to return to the eye in white and colored sparkles. The virtual facets need to appear in sharp focus in order to produce the optimal display of fire and brilliance. Excessive graining can diminish the crispness of that display, even if the stone is perfectly cut.

Many consumers today are aware that cut quality has the greatest impact of all the 4Cs on diamond beauty. After all, rarity and durability are important attributes but diamond optics – fire and brilliance- is where the magic is. Much more is known today about the proportioning and facet precision required to optimize cut quality. But optimized light performance also depends on the material being fully transparent. Otherwise, even the most finely tuned system of tiny mirrors will be unable to reflect and refract light to its full potential. Think of your reflection in a pristine mirror versus a mirror with a very slight film on it. You can still see yourself, but the crispness is lost. And it may only become obvious when you clean the mirror and see your reflection in high definition.

A transparency deficit can be a very subtle effect. And to someone not well versed in evaluating diamonds, it may go unnoticed. Yet, the stone will not be capable of producing full fire and brilliance. The stone may look good initially when clean and in good lighting, but may go glassy or frosty very quickly with a little bit of film buildup from daily wear.

Haziness will be more pronounced in directional light as opposed to diffuse lighting. Therefore a good way to spot a transparency problem is to shine a bright light through the side of the stone while observing it from the face-up direction. A stone with an appreciable transparency issue will look decidedly milky in this scenario.

Another good technique is to compare a stone known to have full transparency with the test stone in question in a variety of lighting scenarios, including the test described above. It’s important to recognize that like fluorescence, transparency is a matter of degree. It is not simply a yes/no question. A very slight deficit may be negligible in terms of the real world consequences. But shoppers looking for the best in cut quality and light performance should be assured that the diamond does not have significantly compromised transparency.

Another example of a material that is generally considered to be transparent is water. Of course ,if water contains sediments or other impurities it will not be fully transparent. This would be analogous to diamonds with inclusions. But transparent water can sometimes form layers of different temperatures called thermoclines. And the difference in temperature changes the optical properties between the layers enough to disrupt light transmission and cause negative optical effects. This is similar to what happens when the carbon lattice of a diamond is heavily striated. The image below shows divers at a thermocline. The diver on top right is swimming just above the temperate gradient and the diver at lower left below it. You can see the sharp focus of the diver’s body that is above the thermocline and the distortion of the image of the diver below. Researchers sometimes refer to this visual effect in lab grown diamonds as a “roiled” appearance or the “scotch and water” effect.

As fundamental as transparency is to the processing of light by a polished diamond, it is bewildering that the best-known gemological laboratories do not directly report on this critical quality factor. For any clues to a potential transparency issue you need to know how to read a GIA report, putting together certain information under Comments with the clarity grade and specific clarity features. This level of interpreting a lab report is beyond the experience of most diamond shoppers.

GIA has hinted at the prospect of someday adding a transparency assessment to their reports using a method employed in their 2021 study on diamond fluorescence. Strongly fluorescent stones are known to sometimes be hazy or milky due to transparency issues. Using a method to quantify changes in contrast as a measure of transparency, GIA determined that fluorescence alone does not cause transparency issues, and that those fluorescent diamonds that are milky are so mainly as the result of structural issues and light scattering inclusions such as graining and twinning lines. It is thought that strong fluorescence might aggravate transparency issues caused by these and other defects. Interestingly, the study revealed for the first time that strong fluorescence does cause a small loss of contrast in a diamond. Since contrast is a necessary component of brilliance, this could be part of the impression that many people have that fluorescence has negative impacts on a diamond’s appearance. Loss of contrast could result in loss of definition of virtual facets (the reflections we actually see which are far more numerous than the physical facets on the diamond), and an appearance that is somewhat flat compared to a diamond with no fluorescence.

“We observed that stronger fluorescence produces some contrast loss in gem diamonds. However, our results show that this contrast loss from strong fluorescence does not by itself cause the milky or hazy appearance observed in some diamonds by the trade. Atomic-scale defects in the diamond structure or nano-inclusions appear to be the main causes of the milky or hazy appearance. The occurrence of strong fluorescence in combination with these features may cause a diamond to appear even milkier or hazier, but we saw no evidence that strong fluorescence alone produces noticeable haziness in diamonds that did not already contain light-scattering structural defects or nano-inclusions. The bulk contrast method presented here may also serve as a reliable way to quantitatively evaluate the effect of contrast loss on apparent transparency in future diamond grading processes. We are reviewing these quantitative and semi-quantitative results to see how they might be included in GIA grading reports. We believe this new information will help to create more accurate information in the trade and ultimately allow consumers to select diamonds based on unbiased scientific and aesthetic factors.”

There is at least one laboratory taking on this task on behalf of the consumer – The Diamond Certification Laboratory in Australia (DCLA) performs a transparency assessment as part of their grading procedures. Their methodology involves an assessment of the diamond’s clarity profile in the context of their database of similar diamonds along with a meticulous visual inspection by their trained gemologists. Their long-range goal is to develop a large enough database to train AI on determining transparency grades in the future. *The following information comes from the DCLA website.

“Transparency is a critical yet often underexplored parameter in diamond grading, especially in stones of otherwise high clarity and quality. Subtle features such as internal graining or microscopic clouding may appear negligible under routine observation, yet under magnification they often reveal distortions in the diamond’s crystal lattice. The diamond lattice, composed of a tetrahedral network of sp³-bonded carbon atoms, is normally responsible for diamond’s exceptional hardness, high refractive index (n ≈ 2.42), and strong light return. However, any irregularities in lattice formation—whether from strain, growth zoning, or inclusions—can disrupt the uniform propagation of light through the medium.”

• Inclusions and Blemishes: A diamond with high transparency is typically free from inclusions or blemishes that may hinder light passage. The absence of internal flaws, such as clouds or graining, contributes to its clarity. Transparency grades range from Excellent to Poor, with higher grades reflecting superior clarity.
• Cut Quality: The cut of a diamond significantly impacts its transparency. A well-cut diamond features symmetrical shapes and well-proportioned facets that facilitate optimal light entry and exit at the correct angles. This ensures that light performs effectively, enhancing the diamond’s brilliance.
• Clarity: A diamond with good clarity lacks internal and external imperfections that can interfere with light transmission. High-clarity diamonds allow light to pass through unobstructed, further boosting their transparency.
• Color: The color of a diamond also plays a role in its transparency. Diamonds with a high level of colorlessness (near colorless) permit more light to pass through compared to those with noticeable yellow or deep colored tints. Fancy colored diamonds, particularly in deep intensities, may also affect the overall perception of transparency.

It is interesting to note that the DCLA methodology considers cut quality integral to transparency, as opposed to an independent factor. Thus, it is not possible for a diamond to exhibit optimal light performance OR to appear fully transparent in the absence of a precision cut.

To a certain extent clarity grades for natural diamonds are rarity grades, at least in the top range. The differences in performance and beauty in the VS1 and better grades is minimal (assuming no significant structural defects), while price is significantly impacted. Beginning at about VS2 beauty and performance start to be impacted by certain factors, such as inclusions that are visible to the naked eye and by transparency deficits. A high percentage of value shoppers regularly look for eye-clean Si diamonds. Some are not only eye-clean but may also have very clean stone plots on laboratory reports. Such stones are often thought to be “unicorns” because they are priced low and look good on paper, at least to those not well versed in the finer points of reading a GIA report. But often these stones have significant transparency deficits, and even with the best cut grades will have diminished light performance.

Without understanding transparency and its sometimes subtle but significant role in diamond beauty, a consumer may not know the whole story about the stone they are considering. It will be a major benefit to the consumer market when laboratories routinely report on the level of transparency for a more comprehensive grading of diamond quality.