Category: Diamond 101

Phosphorescence is a special glow effect seen on some lab-grown diamonds. When you take away the ultraviolet light source, these diamonds can still keep shining for a while. This is a common and interesting feature of many lab-grown stones, especially those made with the HPHT growth method. For daily wear, this trait almost does not change how the diamond looks. However, it is a key clue for gem identification. As a buyer, you just need to learn about this feature and pick diamonds according to your own likes.

What is Phosphorescence?

First, we need to tell apart fluorescence and phosphorescence, because many people mix them up.

Fluorescence means a diamond gives out visible light when exposed to ultraviolet rays, such as UV check lights or strong sunlight. The light will disappear right away once the UV source is removed. You can find fluorescence on both natural diamonds and lab-grown diamonds, so it is really normal.

Phosphorescence works differently. The diamond continues to emit light long after the ultraviolet light is gone. The glowing time can be just a few seconds, or even last for several hours. This phenomenon is extremely rare in natural diamonds, but it is quite typical for certain lab-grown diamonds.

Here is a easy example to help you understand. It is just like the hands on a glow-in-the-dark watch. After absorbing light, the watch hands will glow softly in the dark for some time. That is exactly how phosphorescence acts

Why do lab-grown diamonds have phosphorescence?

The appearance of phosphorescence mainly connects with the diamond’s growing technology and tiny defects inside its crystal structure.

The most common cause comes from the HPHT production method. During the HPHT diamond growing process, workers often use catalysts that contain boron. These boron elements help the diamond form faster and also adjust its color.

After boron atoms enter the diamond crystal lattice, they will combine with other defects inside the stone, like nitrogen atoms. The most well-known combination is called B-N pairs, short for boron-nitrogen pairs.

When these B-N defects get excited by ultraviolet light, the electrons inside will jump to a higher energy state. These electrons take a very long time to return to their normal state. While they slowly settle down, they release energy in the form of visible light, and this creates the phosphorescence we see.

Things are a little different for CVD lab-grown diamonds. Most CVD diamonds do not contain boron, so strong phosphorescence hardly show up on them.

Even so, some other small lattice defects may still form during CVD production, such as empty atomic spaces or silicon impurities. These flaws can lead to weak phosphorescence too. Still, the glow usually fades very quickly and comes in different colors.

What does phosphorescence look like?

When a diamond shows phosphorescence, its glow has clear features.

In terms of color, blue-green and yellow-green are the most common types. The exact color depends on what kind of crystal defects exist inside the diamond.

As for lasting time, most phosphorescence only stays for several seconds to a few minutes. Only a very small number of diamonds can glow for hours, but their brightness drops fast from the very start.

How does phosphorescence affect a diamond’s look and value?

We can look at this question from two different sides.

For daily wearing, phosphorescence has no bad effects at all. Under normal natural light or indoor lights, you cannot see the glow. It will not change the diamond’s original color, brightness or sparkle. You will never see your diamond glow on its own when you stand under sunshine or restaurant lights. There is no need to worry about it ruining your wearing experience.

In some special situations, phosphorescence can become a unique highlight or a point that people care about.

Many people think this hidden glowing feature is really cool. For example, when you stay in a nightclub or turn off the lights at home, your diamond will send out soft mysterious light. It makes your jewelry one of a kind.

For professional gemologists, obvious and long-lasting blue-green phosphorescence is an important identification sign. It strongly suggests the diamond is made by HPHT technology. Since natural diamonds with such trait are almost impossible to find, this is a useful testing hint.

In rare cases, super strong and long-lasting phosphorescence may bring strange feelings. If you walk from bright sunlight into a dark place like a movie theater, the glowing diamond may catch other people’s eyes. A small number of customers do not like this unnatural glow and feel uncomfortable with it.

It is important to know that phosphorescence is just a normal physical effect formed during crystal growth. It is not a quality flaw. It will not make the diamond easier to damage, nor ruin its beauty for daily use.

If you do not like glowing diamonds, you can talk to the sellers and ask for stones without phosphorescence. Reliable jewelry stores have professional tools to test and tell you this feature clearly.

Top grading labs like GIA and IGI will check phosphorescence when grading lab grown diamonds. They will write down the result in the Comments section on the official certificate. You may read sentences like “Phosphorescence is present” or “No phosphorescence detected” there.

On the other hand, if you find this glowing effect fun and special, you can also choose such diamonds on purpose. Let this unique feature make your jewelry stand out from others.

HPHT is one of the earliest invented ways to make lab-grown diamonds, and this technology is already very mature. HPHT stands for High Pressure High Temperature, which we call high temperature and high pressure method for making cultivated diamonds. This tech copies the natural environment where real diamonds form deep inside the earth’s mantle. The machine used to make synthetic diamonds looks just like a huge pressure cooker. Inside a tiny sealed chamber, workers put carbon materials under extreme pressure, around 5 to 6 GPa, and high temperature from 1400 to 1600 degrees Celsius. Under such conditions, carbon atoms will rearrange and turn into brand new diamonds.

The production steps of HPHT diamonds

First of all, we need to get all raw materials ready. Inside a carefully built press chamber, we place high purity carbon sources, most of them are graphite, and a very small diamond seed crystal.

Next step is adding catalyst. People put metal catalyst into the chamber. It is usually alloy mixed with iron, nickel and cobalt. These metal materials can lower the energy needed for diamond to form under high heat and pressure. It helps graphite change into diamonds much easier.

Then we create the extreme growing environment. Turn on the big press machine. The chamber will get pressure as strong as the place more than 150 kilometers under the ground. The pressure keeps at 5-6 GPa, and temperature stays between 1400℃ and 1600℃ all the time.

After that comes melting and crystal growing. In this tough environment, the metal catalyst melts first, and then it dissolves the graphite carbon. The dissolved carbon atoms move through the hot liquid metal, and stick on the small diamond seed which has a little lower temperature. Little by little, the crystal grows bigger and bigger.

When the growth is done, we start to cool down and release pressure. The whole growing process can take several days or even weeks. After temperature and pressure back to normal, we take out the rough diamond crystal. At this moment, the raw diamond is always covered with metal catalyst on its surface.

The last work is cutting and polishing. Workers clean off all the metal stuff on rough diamonds. Then professional craftsmen cut and polish the crystals carefully. Finally, we get finished diamonds ready to sell.

Features of HPHT diamonds

The rough crystals of HPHT diamonds usually have a special shape, mixed with cube and octahedron form. It is totally different from natural diamonds which are mostly octahedron, and CVD diamonds that look like flat plates.

HPHT tech is really good at making colorless diamonds in D-E-F color grade, and blue diamonds too. The blue color comes from boron element inside the crystal. It can also produce many other fancy colored diamonds, such as yellow and yellow-green ones.

Because metal catalyst is used during production, tiny metal inclusions can be found inside some HPHT diamonds. If you look under a microscope, these inclusions are small dark dots with metal shine. What’s more, most of these diamonds have magnetism. You can use a strong magnet to test it, the stone will be attracted.

Famous gem labs like GIA and IGI can do full identification for finished HPHT diamonds. Every official certificate will clearly mark it as Laboratory-Grown diamond, and note HPHT as its growing method. The metal inclusions and unique fluorescence pattern are the main clues for experts to tell HPHT diamonds apart.

HPHT and CVD technology comparison

There are two main technologies to make lab diamonds in the market, HPHT and CVD. They work in different ways.

HPHT imitates the underground nature environment. It melts graphite carbon with high pressure and heat, then makes it recrystallize into diamonds. CVD uses gas as raw material. Carbon atoms separate from gas and slowly build up on diamond seeds.

Their crystal shapes are not the same. HPHT diamonds are cube or octahedron shape. CVD diamonds are flat and board-like.

In color performance, HPHT works best for colorless diamonds, blue diamonds and fancy yellow diamonds. Most original CVD diamonds have brown or grey tint. People often use HPHT treatment later to remove the bad color and turn them colorless.

For inclusions inside stones, HPHT has metal inclusions with magnetism. CVD diamonds contain graphite or other non-metal impurities, and they do not have magnetism at all.

When it comes to usage, HPHT is widely used for small diamonds, colorless stones and colored diamonds. It also acts as a color improvement process for CVD diamonds. CVD is more popular for big size diamonds and fancy shaped diamonds in the market.

HPHT is a steady and well-developed lab diamond technology. It gains great reputation for nice colorless and colored diamonds. When you plan to buy one, please check the official certificate first. Learn about its possible magnet feature, and choose honest and reliable sellers. In this way, you can get a satisfying HPHT diamond.

HPHT lab-grown diamonds are real diamonds. They share the same chemical composition, physical property, hardness and optical effect with natural diamonds. They are not diamond simulants like moissanite or cubic zirconia.

The price of HPHT diamonds is much lower. It is only one sixth to one thirteenth of natural diamonds in same grade. Even people with limited budget can afford big and high quality diamonds now. Always ask sellers for certificates from GIA or IGI. The papers can prove the diamond’s real 4C standard and confirm it is lab-grown.

Some HPHT diamonds have weak magnetism because of inner metal inclusions. You can test it with a strong neodymium magnet. This is a normal feature of HPHT diamonds, not a quality defect. But sellers should tell buyers this point clearly before deal. CVD diamonds never have magnetic property.

HPHT has great advantages on cost and efficiency when producing small melee diamonds and colored diamonds, especially blue and yellow ones. A large number of colorless CVD diamonds on sale today are processed by HPHT heat treatment to get rid of unwanted color tone.

CVD lab-grown diamond is one of the most popular technologies for making lab diamonds right now. It is a kind of gem made by modern advanced technology, and it is exactly the same as natural diamond in all aspects. It gives people a good choice to get bigger and better quality diamonds at a much lower cost. When you plan to buy one, remember to pick reliable sellers and check official certificates carefully.

CVD stands for Chemical Vapor Deposition, its Chinese name is chemical vapor deposition method. This tech can create a natural-like environment inside a lab, and let diamond grow layer by layer. Many people describe it as growing diamonds like planting crops. Workers put a tiny diamond seed into a sealed chamber. Then they fill the space with carbon-rich gas such as methane, and add energy with microwave equipment. The carbon atoms in the gas will fall down on the seed little by little, just like snowflakes. After a period of time, these atoms build up and turn into rough diamond crystals.

The production steps of CVD lab-grown diamonds

First step is preparing the diamond seed. People place a thin slice of natural diamond or existing lab-grown diamond as the base seed, and put it into a sealed vacuum growth chamber.

Next is gas injection. The chamber will be heated up to a high temperature, around 800 degrees Celsius. Then workers pump carbon-containing gas like methane and hydrogen gas into the closed space.

The third step is gas ionization. Microwave energy is used to ionize the mixed gas, turning it into plasma with high chemical activity. In this state, carbon atoms are separated from methane molecules completely.

Then comes carbon atom deposition. Those free carbon atoms float in the chamber and settle on the diamond seed slowly, one layer after another just like fine dust.

After that is crystal growth. All carbon atoms arrange themselves following the standard diamond crystal structure. They keep stacking day after day. This process usually takes several weeks or even longer, until a complete rough diamond crystal is formed.

The last step is cutting and polishing. The rough diamond is sent to professional cutting factories. Experienced craftsmen cut and polish it in the same way they deal with natural diamonds. Finally, the raw stone becomes finished diamond products.

Main features of CVD diamonds

CVD diamonds usually have high clarity. The whole growing process is easy to control, so there are fewer metal catalyst inclusions inside. But they may have special inclusions, such as black carbon or graphite spots.

Most raw CVD diamonds show light brown or grey color tone. Most of them need extra HPHT high pressure and high temperature treatment later. This common industry process can turn them into colorless diamonds graded D to F, or near-colorless ones from G to J. This kind of post-treatment is widely used and well known in the diamond business.

This technology is good for making flat-shaped diamond crystals. It is perfect for producing fancy shapes like princess cut, cushion cut and radiant cut diamonds. However, if people want to make large round brilliant diamonds with CVD method, it will waste more raw materials.

Professional gem labs such as GIA and IGI can fully identify CVD lab-grown diamonds. They will issue formal grading certificates, which clearly mark that the diamond is lab-grown and produced by CVD technology.

CVD VS HPHT, two main lab diamond technologies

CVD and HPHT are the two leading ways to create man-made diamonds in the market. Their working principles are quite different.

For CVD technology, carbon atoms separate from gas and deposit on the seed crystal. It is like growing diamonds from gas. As for HPHT, it simulates the deep underground environment of the earth. Workers melt carbon sources like graphite under extreme high temperature and pressure, then let carbon recrystallize into diamonds.

CVD diamonds mostly form flat plate-shaped crystals. HPHT diamonds often appear in cube or octahedron shapes.

In color performance, original CVD diamonds often carry brown or grey shades. They almost need post treatment to improve color. HPHT diamonds tend to show blue or yellow hints because of boron and nitrogen elements inside, while it is also easy to make colorless HPHT diamonds.

In terms of clarity, CVD diamonds generally have better clarity, and their main inclusions are carbon spots. HPHT diamonds may contain metal catalyst inclusions. Some of them can be slightly attracted by strong magnets.

Nowadays, CVD takes a larger share in the market, especially for medium and large sized diamonds. HPHT is a more traditional method. It is widely used for small diamonds and color enhancement treatment.

It is very important to know that CVD lab-grown diamond is real diamond. It shares the same chemical composition, physical features and optical performance with natural diamonds. Both are pure carbon cubic crystals. Its Mohs hardness reaches level 10. The fire and sparkle of CVD diamonds are no different from natural ones at all.

The price of CVD lab-grown diamond is only one sixth to one fifteenth of natural diamond with the same 4C grade. It has an extremely high cost performance. When shopping, always choose diamonds with official certificates from GIA, IGI and other authoritative organizations. These certificates can prove the true quality of 4C standards, and clearly label the stone as Laboratory-Grown diamond.

Almost all colorless CVD diamonds sold on the market have gone through HPHT post-treatment to remove brown color. This is a normal and acceptable optimization method in the industry. Honest and trustworthy sellers should take the initiative to tell customers about this treatment. On GIA certificates, there will be notes saying Indications of post-growth treatment to record this situation.

As a high-tech product, producing CVD diamonds causes much less damage to the environment compared with traditional diamond mining. It also avoids the problems of conflict diamonds. It fits the eco-friendly and ethical consumption ideas of modern people.