Here’s a lab diamond purchaser posting on Reddit asking why their new I color lab diamond is so gray compared to their other lab diamond.

Here’s a lab diamond purchaser posting on Wedding Bee that their lab diamond had a “weird” faint brown hue compared to their natural diamond.

The first step of CVD diamond growth is preparation of the seeds. Great care needs to be taken to ensure the seed is as perfect as possible, as the quality of the seed determines the quality of the diamond. The seeds are strategically arranged on the growth platform in the reactor, and the all the air is sucked out of the reactor. A high power microwave generator is activated, and ultra pure methane is fed into the reactor. The microwave energy causes a plasma to form above the seeds, which turns the seeds a bright red color.

In the plasma, the methane begins to break apart, and the carbon from the methane attaches to the diamond seeds, growing vertically, atom by atom, for weeks at a time.  On the edges of the diamonds, black polycrystalline material begins to form. That black material is removed with a laser, and the gemstone can be cut from the rough CVD.

Approximately 80% of CVD diamonds undergo post-growth treatment, which is a final stage of refinement. The CVD diamond is loaded into an HPHT press for a quick 15-30 minute heat treatment to improve the crystallographic structure and appearance of the diamonds.

Video courtesy of ALTR Created Diamonds

Since 2009, GIA has seen a steady increase in CVD grown diamonds to approximately 80% of the total lab diamond grading reports in the latest published data.

Why is CVD outpacing HPHT? It’s a more efficient technology. The equipment is substantially smaller and does not require a specialized facility. It generally produces Type IIa completely inert under SW and LW light and does not test as moissanite (for the most part) on cheap diamond handheld diamond testers. It’s also possible to observe diamond growth and restart growth when problems arise

What are the drawbacks and limitations of CVD? It has slower growth rates than HPHT, requires expensive seeds from third parties, and quality can vary. CVD is more prone to crystal strain, which can cause blurriness. It’s also less economical than HPHT for melee sized diamonds, also known as accent diamonds. Most HPHT melee is high color, which means pairing a much warmer CVD with bright white side stones can be visible to the naked eye.

These are all G color CVD lab diamonds. As you can see the color tinges can vary in hue, saturation, and severity, even within the same color grade.

Small amounts of nitrogen allow you to grow diamonds 200-300% faster. However, the faster you grow CVD diamonds, the more that can go wrong. As the atoms stack on top of each other, sometimes there are stacking faults, or glitches in the matrix of carbon atoms. One of the issues that can arise are voids of missing carbon in the crystal, like bubbles trapped in ice.

Nitrogen can also give CVD diamonds a brown tone. Even though there is far less nitrogen in a CVD diamond than an average natural diamond, the optical consequences of said nitrogen can be more obvious to the naked eye in CVD than in natural. Single substitutional atomic nitrogen is actually more apparent and brown in tone than aggregated nitrogen.

Brown can also mean brownish pink, and this is primarily caused by nitrogen vacancy defects resulting from low pressure, high temperature treatment.

If the color of as-grown CVD diamond is brown, it can be made colorless through post growth treatment. That treatment can remove the voids from the crystal structure and also aggregate the nitrogen atoms, reducing the amount of brown they create.

A more extreme example of untreated (left) versus treated (right) CVD.

The most prevalent color tinge is gray and it can have several causes. It may also be accompanied by lifeless material. Gray is also possible in natural diamonds, but for the most part not at the same level as in lab grown and certainly not for the same reasons. Gray tinge can be seen as high as D color.

The diamond on the left has a gray tinge over the diamond on the left.

Silicon trapped in the crystal can cause gray tinges. Silicon is very rare in natural diamonds, but common in CVD diamonds. Most frequently, the silicon is from the inside of the quartz window of the CVD reactor literally burning off into the vacuum chamber from the plasma.

Adding nitrogen speeds up the diamond growth, but it leaves an undesirable brown tinge. You can compensate for the nitrogen by adding boron to your recipe – the pairs of atoms find each other and cancel out the properties of each other. But if you add too much boron, the nitrogen is overruled, and the diamond can present with a gray tinge/tone.

Strain presents as blurriness in the crystal, and it looks almost like you can never get it fully clean. The closest approximate natural characteristic is graining.

You can see the “shimmer” on the CVD rough as the light hits the strain.

Strain is not measured as a binary but rather exists on a scale of good to bad. There’s little to no consensus in the industry as to what is light or faint and what is strong. Strain is not a clarity setting feature, meaning it does not appear in inclusion maps or change clarity grades. All of the above diamonds are VVS in clarity.

Another reminder that lab diamonds are not identical to natural and can be detected! It’s worth noting that HPHT has little to zero strain when viewed under cross polar filters.

The quality of seeds can vary significantly and build either a strong or weak foundation for CVD diamond growth on top. Seeds viewed through an x-ray (at right) can show faults or uneven surfaces.

One CVD reactor can have varying degrees of quality seeds. The quality of the material of the grown CVD can vary significantly for the same grower and even in the same growth run!

Seeds deteriorate with each use and the diamonds grown on top of them get worse and worse. Replacing seeds is very expensive. Many growers will use and reuse seeds far past their prime.

As strain builds, it can eventually explode, almost like a volcano, and form into polycrystalline inclusions. That does impact clarity. The diamond on left is CVD and diamond on right is HPHT. Both got the same clarity grade of I1.

Image at left courtesy of GIA

The diamond on the bottom has over 8 growth cycles and is more heavily striated. The diamond on the top has a cleaner crystal material.  Note the difference in contrast between the two diamonds.

Every time a CVD reactor is stopped and restarted, contaminants get into the plasma, which cause the rings of diamond growth to look dark gray, further contributing to color tinge face up.

Approximately 80% of GIA graded CVD diamonds are HPHT treated. Why? Proper HPHT treatment makes the diamonds better. Period.

We believe the resistance to post growth treatment is a hangover effect from the natural diamond market where there’s legitimate concern about undisclosed manmade treatment being done to a natural product. But in lab grown, the product is manmade throughout the entire process. And what one grower considers treatment another may consider simply part of the growth process.

Untreated CVD has some flaws. It’s less durable and has more crystal material issues like brown and strain. Post-growth treatment effectively heals brown and strain. Not all treated CVD is good, and not all of it is bad.  

Small diamond seeds are placed at the bottom of a growth cell. Catalytic metal material is placed on top. Graphite is added into a gasket. This growth cell is then placed at the center of a hydraulic press where six different anvils exert pressure on six different sides (which is why it’s called a cubic press). The press is heated to 1500 degrees Celsius and subjected to 1 million psi of pressure. The graphite is heated to the point where it melts into liquid carbon. A convection is created. As the press cools, the carbon affixes to the diamond seeds and grows atom by atom, crystalizing into diamond. The seeds with which the diamonds are grown in the rough HPHT are visible upon removal.

Blue tinge in HPHT diamonds is a very common crystal defect. This is due to trace boron that is intentionally added during the growth process to compensate for nitrogen. Measurable levels of boron are considered Type IIb. 75% of HPHT diamonds submitted to GIA for grading are Type IIb, but that doesn’t necessarily mean that they all have blue tinge! Type IIb is more likely to have blue tinge, but it’s not definitive.

In the video above, the D, F, and H color HPHT diamonds have a blue tinge, but notice that the E color does not. Color is measured for the presence of any color, not just yellow.

While there certainly exist blue natural diamonds, they are exceptionally rare, making up less than 0.1% of the gemstone market and fetching significantly higher prices per carat than traditional natural or lab diamonds. It is not feasible that a member of the public would “accidentally” purchase a large Type IIb natural diamond. So having a blue tinge diamond may make someone question whether the material was diamond or something else like aquamarine or moissanite.

While a blue tinge may be a consumer’s personal preference, it is technically a crystal defect.

Both “F” color HPHT lab diamonds. The bottom diamond has a blue tinge.

For years, HPHT diamonds were yellowy orange due to the presence of nitrogen. Added boron helps to compensate for the nitrogen. In some cases you can end up with excess nitrogen, just the right compensation, or too much boron. While this practice also exists in CVD, it’s much more common and at a more significant level in HPHT.

Left to right: Substitutional nitrogen atom; boron compensating for nitrogen; excess boron.

Gray HPHT can have nice material but a slate color

Phosphorescence can be seen in some HPHT diamonds after exposure to long wave light. Phosphorescence can have zoning just like boron.

Here are four HPHT diamonds. On the far left, there’s one that has a blue tinge and one that has a yellowy tinge. The yellowy tinge emerald cut is Type IIb despite not having a blue tinge. On the right there’s a blue tinge as well as a yellowy tinge HPHT stone.

The four diamonds are moved from the paper tray to a perforated tray. After just a few seconds of long wave UV light using a handheld light from Amazon (you can buy for $5) being applied, the main light is turned off.  The two diamonds on the left had very strong and moderate phosphorescence and the two diamonds on the right are inert, with no reaction to long wave UV light.

Phosphorescence is not just seen in blue tinge or avoidable in yellowy tinge. It is connected to boron at an atomic level, not an optical one.

The office light that activated phosphorescence.

Phosphorescence is not disclosed on a grading report and is only something that can be tested for in person. It’s also not a great “surprise.” Imagine walking into a dark movie theater and suddenly seeing your diamond glow, without warning. Consumers believe that lab diamonds are chemically identical to natural diamonds, but the number of natural diamonds that have phosphorescence under normal indoor lighting? Fewer than 0.1%. The presence of glow-in-the-dark diamonds has the potential to erode consumer trust in HPHT lab diamonds if consumers don’t know that it exists. Medium to strong phosphorescence can also make a lab diamond look cloudy/hazy in low light environments.