HPHT vs CVD Lab Grown Diamonds
Diamonds are pure crystals of carbon: the only gem formed from a single element. But pencil-soft graphite is carbon too. What gives diamond its extraordinary properties is its crystal structure. The carbon atoms in diamond are bonded closer together than in any other substance on Earth. (You’d have to journey to a neutron star to find atoms closer together!) This unique structure gives diamonds exceptional hardness, unrivaled brilliance and scintillation, and high thermal conductivity. Forming the exceptionally strong bonds of diamond crystals requires a lot of energy. In the earth, diamonds formed a billion to three billion years ago deep beneath the Earth’s crust in the base of the continents, where the pressure and the temperature are extreme.
That’s why it took scientists so long to develop a way to grow actual crystals of diamond in a lab at the Earth’s surface. The first experimental diamond crystals were produced in 1955 but gem-quality man-made diamonds didn’t reach the jewelry market in large quantities until the last decade due to scientific advances that made producing beautiful diamonds in the lab more affordable. There are two ways to grow diamonds in the laboratory: High Temperature High Pressure Growth Process, or HPHT, and Carbon Vapor Deposition or CVD.
What are HPHT Diamonds?
The first way scientists grew diamonds was the HPHT process, which reproduces the conditions deep in the earth. Huge capsules called diamond presses heat carbon atoms up to 1300-1600C and squeeze them with 5-6 gigapascals of pressure, the weight of 80 elephants stepping on your toe. Suspended in a liquid metal catalyst under that tremendous heat and pressure, HPHT-grown diamond slowly crystallizes on a cooler diamond seed.
Today, HPHT lab diamonds are available in colorless, near-colorless, and fancy colors in every size and shape. The largest HPHT created diamond ever grown is a crystal weighing 150 carats. (Although that’s much smaller than the largest mined diamond, the 3,106 carat Cullinan Diamond, it’s a huge advance in technology.) Of course, most of the HPHT lab grown diamonds on the market are much smaller than that. HPHT lab created diamonds are readily available and affordable in sizes less than a half-carat, where they dominate the market. Because the HPHT process uses a lot of energy, it is more expensive to grow larger sizes.
HPHT lab grown diamonds do have some characteristics that are unique. They sometimes have dark metallic inclusions from the catalyst used in the growth process that aren’t seen in any other diamond origin. They can even be magnetic. They are usually Type IIa or IIb. Some HPHT created diamonds contain trace amounts of boron from the growth process which can give them a faint blue cast and increase their electrical conductivity. (Diamonds normally don’t conduct electricity.) In rare cases they can test as “not diamond” on an electrical conductivity tester. They also have an unusual crystal shape: a cuboctohedron shape that shows both cube and octahedral faces, growth features that can be used to identify them when viewed under deep UV fluorescence.
What are CVD Diamonds?
The CVD process is an exciting innovation in diamond growth that was first proposed in the 1980s. It took decades to commercialize the process: the first gem-quality CVD diamond weighing one carat wasn’t produced until 2010. But today, CVD is the most popular way to produce center diamonds from 1-3 carats. The largest faceted CVD lab grown diamond produced so far is 16.41 carats.
In the CVD process, scientists use microwaves to heat methane and hydrogen gases in low pressure to form a plasma, freeing its carbon to deposit, atom by atom, onto a diamond seed. CVD lab diamonds are grown in layers only a few carbon atoms thick until they build up into a diamond cake. Sometimes CVD lab grown diamonds are finished in an HPHT diamond press to improve their color. (Some sellers call this "post growth treatment" but it is essentially the same as the normal HPHT diamond growth process.)
CVD lab created diamonds are Type IIa, the purest type of diamonds chemically. Most CVD lab-grown diamonds are near colorless with excellent clarity. Colorless, near colorless and fancy colors are all available. CVD grown diamond crystallizes in a distinctive cubic crystal habit, with growth in only one direction, that can be used to identify them when viewed under deep UV fluorescence.
Growing diamonds using the CVD process uses less energy and smaller machines. Growers have more control over the process so can produce more consistent quality. Because CVD diamonds use less energy to grow, some producers are carbon neutral. Look for Certified Sustainability Rated Diamonds accreditation from SCS Global Services if you are interested in sustainably grown diamonds.
CVD vs HPHT Diamonds: The Bottom Line
Both CVD and HPHT lab grown diamonds are available in a wide range of sizes, shapes and qualities, just like natural diamonds. They are all cut and polished in the exact same way, often in the same factories. Like all diamonds, they are grading according to the 4Cs of diamond quality. You can have the quality of your CVD or HPHT lab created diamond certified by the same labs that grade natural diamonds too.
The different ways diamonds grow result in different rough shapes. Natural diamonds most commonly have octahedral crystals or rounded forms from alluvial deposits. HPHT grown diamonds have cuboctohedral crystals that have both cubic and octahedral faces. CVD lab grown diamonds have tabular shaped crystals with a single growth direction with a rim of polycrystalline diamond on the outside. Although these characteristic shapes are cut away during the polishing process, the crystal growth patterns can be seen by examining the diamond under magnification using an instrument that uses energetic deep UV illumination. No matter what the rough looked like, you can’t tell by looking at a finished diamond whether it was mined, grown by the HPHT process, or grown by the CVD process.
When choosing between HPHT vs CVD lab diamonds, your jeweler’s decision will most likely depend on size and quality. Below a half carat in size, HPHT is the most affordable and available option. From 1-3 carats in size, CVD generally offers the best value and the best quality. Above five carats, HPHT is generally more available. In between these ranges, it will depend on the color and clarity you are looking for in your lab grown diamond.
In the end, it doesn’t really matter whether your diamond was created using the CVD or HPHT process. What’s most important is the quality of the diamond, whether or not the producer is Sustainability Rated, and the style and quality of your jewelry piece. Because both HPHT and CVD are about half the cost of mined diamonds, you can afford to trade up to an exceptional quality that has all the enduring beauty that you expect from diamonds.