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In-Depth
The Technology of Lab-Grown Diamonds
Diamonds take billions of years to form in the earth, but new technology allows for diamonds to be grown in labs in a matter of weeks.
By Shuan Sim
The conventional path of a diamond from mine to market is a time-consuming one. Many months are required for a diamond to be extracted from the earth, tendered to companies that cut, polish and use it in a piece of jewelry, before it is sold to retailers. That is not including the billions of years diamonds take to form under the ground. Today, it could take as little as weeks for a diamond to be grown and purchased by a consumer thanks to diamond-growing technology.
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Since 1797, when it was discovered that diamonds are composed of pure carbon, there have been many failed attempts to create man-made diamonds from carbon. It was not until the 1950s that the first reproducible methods of synthesizing diamonds were discovered. In February 1955, General Electric announced that it had successfully and reliably created diamonds using high pressure-high temperature (HPHT) methods to produce a diamond — with the largest at .15mm, or .00075 carats. Chemical vapor deposition (CVD) was also in use during the 1950s, but the method was only capable of producing diamond films and small polycrystalline diamonds unsuitable for jewelry. In the late 1990s, Apollo Diamond Inc. — acquired by South Carolina–based Scio Diamond Technology Corporation in 2011 — discovered a way to grow monocrystalline diamonds, paving the way for gem-quality CVD diamonds. HPHT diamonds comprised the bulk of early lab-created diamonds, but they were only suitable for industrial use. The first gem-quality diamond was not seen until the 1970s. However, many of those diamonds were small and were yellow or brown, although they could be treated to improve the color. “It was really only in the 2000s that we started to see jewelry-quality diamonds become commercially viable,” says Eric Franklin, president of AOTC Inc., a Michigan-based producer of HPHT diamonds. Even today, growing white or colorless diamonds remains a challenge, as removing nitrogen, which imparts the yellow color in diamonds, during the growing process is tricky. Big strides in growing technology have brought producers much closer to that goal. While other methods exist, such as using ultrasound or detonation, HPHT and CVD are currently the most prevalently used methods to make lab-grown gem-quality and industrial diamonds around the world.
Growing both HPHT and CVD diamonds boils down to one concept: loose carbon atoms forming around what is called a diamond seed. These seeds can be natural or lab-grown and are essential for instructing carbon atoms in the formation of diamonds. Loose carbon atoms organize themselves to resemble the arrangement of carbon atoms in the diamond seeds — had a graphite seed been used, the resultant growth would be graphite and not diamond. HPHT seeds resemble grains and CVD seeds resemble wafers. A diamond grows from the seeds and the seeds are cut off at the end of the process.
Natural diamonds are typically formed about 90 miles to 120 miles underground, exposed to temperatures of approximately 1,600 to 2,400 degrees Fahrenheit and 4.5 to 6 gigapascals (GPa), a unit of pressure. HPHT systems seek to replicate those conditions to create diamonds. The common feature of all HPHT presses is that the machines exert high continuous pressure and heat on a synthesis capsule. The capsule contains diamond seed grains, solvent metal, highly pure carbon and other metals to remove nitrogen for whiter diamonds or to impart color during growth. The high pressure and heat melt the solvent metal, which acts as a catalyst to speed up the diamond formation. The molten metal in turn dissolves the carbon, which then settles upon the seeds and grows as diamond. There are currently two major kinds of presses available for making diamonds: two-anvil types and multianvil types.
Two-Anvil Presses:- The belt press was one of the earliest HPHT setups for creating lab-grown diamonds. It features two anvils — one from above and one from below — applying pressure against a capsule held in a reinforced cylindrical inner cell. Belt presses, known to be bulky, are still in use today in modern HPHT diamond production on a much larger scale.
- An update of the belt press includes the use of a toroid device, sometimes also known as a toroid press. This disc-like device is more compact and allows pressures of up to 10 GPa to be achieved without cracking the specimen, unlike traditional belt presses, which usually run at 6 GPa. Toroid presses can process multiple seeds at once.
Multianvil Presses:- First seen in Russia around 1990, the multianvil press was known as the “split sphere” or BARS press, which is an acronym of its Russian name. It involves eight outer anvils exerting pressure on six inner anvils, which in turn press against the capsule. A disc-type barrel encapsulates the outer anvils and high pressures are achieved by heating hydraulic oils contained within. The BARS press is capable of pressures averaging 10 GPa, and is considered the most compact of all the presses. However, BARS systems can only grow one diamond at a time.
- Another multianvil press is the cubic press, where six pistons simultaneously compress upon the capsule. A cubic press is typically smaller than a belt press, and can achieve the pressures and temperatures needed to create diamonds faster. Cubic presses are capable of growing multiple diamonds at a time.
Improvements in technology have allowed HPHT producers to reliably grow diamonds, on average, at sizes between 1 carat and 1.25 carats, though larger sizes are possible. According to Franklin, it could take as little as several days to grow 1 carat, and up to several weeks for sizes up to 4 carats. Colorless stones also take longer, and the faster the growth, the yellower the end result. The largest colorless HPHT diamond reported to date is a 10.02-carat square emerald-cut stone, fashioned from a 32.26-carat rough grown by New Diamond Technology (NDT) in Russia. The lab-grown diamond, certified by IGI Hong Kong, was grown in less than 300 hours.
Growing diamonds using the CVD method involves the use of high-energy microwaves energizing methane gas containing carbon and hydrogen atoms in a chamber containing flat diamond wafer seeds. At a certain point, the gas turns into a plasma state, whereupon the hydrogen atoms are free to escape, leaving the heavier carbon atoms behind. The carbon atoms then fall to the bottom of the chamber and are arranged into diamond structures on the wafer. Multiple diamonds can also be grown simultaneously in the growth chamber. The CVD method has greater control over eliminating nitrogen content, resulting in colorless diamonds. Other elements, such as boron to make diamonds blue, can be easily added, making it easier for colored diamonds to be grown as is, instead of using post-growth treatment processes to change a diamond’s color. The diamond wafers can be of any shape and the surfaces of the seeds have to be thoroughly free of inclusions or impurities, which will affect the growth, says Jerry McGuire, president and chief executive officer (CEO) of Scio Diamond Technology Corporation, a CVD diamond producer. The growth of CVD diamonds is linear, and it could take about a month to grow a diamond that can be cut into 4-carat to 6-carat sizes. In theory, there is no limit to how big the diamonds can grow other than practical seed sizes. It is possible to grow a CVD diamond that will end up as a 60-carat stone when cut, but it would require a seed with a surface that large. “Sometimes it is not economical to use super large seeds,” McGuire comments, adding that the market sweet spot is for .75 carats to 2 carats finished, and that is what Scio is currently producing on average.Shapes of Rough Stones- HPHT rough stones resemble natural rough diamonds with a primary difference — they are cuboctahedral — eight triangular faces, six square faces — as opposed to being cubic — six square faces — or octahedral — eight triangular faces. In a natural diamond, the cubic and octahedral faces are developed evenly, resulting in a uniform stone with no cuboctahedral faces. An HPHT rough diamond, because it was created from a seed that is resting on a face while being subject to pressure and heat, is slightly tapered upwards with developed cuboctahedral faces.
- CVD diamonds grow in layers vertically, following the shape of their seeds. If the wafer is circular, the CVD diamond rough grows into a cylinder. If the wafer is a square, the CVD diamond rough will resemble a cuboid. A cylindrical CVD rough could also be grown for the cutting of round diamonds to minimize waste.
Crystalline Structure
- HPHT diamonds have cuboctahedral growth patterns that create an “hourglass” pattern in cut and polished diamonds when examined using ultraviolet light — a telltale sign of an HPHT diamond. Natural diamonds possess triangular-like concentric growth rings, as well as a “tatami” pattern that looks like straw mats in both cut and polished diamonds when examined.
- CVD diamonds and their layered growth create parallel striations in cut and polished diamonds — a phenomenon absent in both natural and HPHT diamonds.
Diamond Type- Almost all HPHT diamonds are type Ib due to their nitrogen content, which imparts the yellow color seen in many lab-grown HPHT diamonds. However, with recent advances in technology, colorless HPHT diamonds are possible, making them type IIa or type IIb. Only .1 percent of natural diamonds are type Ib.
- Most colorless and near-colorless CVD diamonds are type IIa, lacking nitrogen. Only 1 percent to 2 percent of natural diamonds are type IIa.
Magnetism- Some HPHT diamonds can respond to magnets due to ferromagnetic metallic inclusions left behind from the solvents as part of the creation process. The integral nature of solvents in the HPHT diamond growth process means that traces of metal inclusions will be present, though improvements in technology have reduced the amounts and therefore the magnetism.
However, while it is possible for natural diamonds to have metallic inclusions that are also ferromagnetic, they are usually removed during the cutting process, leaving the stones unresponsive to magnets. Since metals are not required for natural diamond formation, such inclusions are comparatively simple to remove.- CVD diamonds are not magnetic at all.
Finished Jewelry- HPHT diamonds are more likely to be cut into melee owing to their smaller average size, according to Lan Yan, director of the National Gemstone Testing Center (NGTC) in Shenzhen, China. Larger HPHT diamonds might have metallic inclusions (see photo below) that the cutter would have to account for in the cutting process to ensure that they are removed or are not visible, points out Franklin from AOTC. “Cutters might have to be aware of them as they can expand and crack the diamond when exposed to heat,” he explains.
- CVD diamonds tend to be cut into bigger sizes, since they are easier to grow in larger sizes. Hill says that he can grow CVD diamonds up to 3.5-carat rounds when cut and 5-carat fancy shapes.
Cost of Production- The cost of producing lab-grown diamonds is considered proprietary information among producers. Franklin estimates that production costs between HPHT and CVD would be similar, and both might possibly be lower than natural diamonds. “Natural diamonds require more hands to get to the consumer and so lab-grown diamonds, with a shorter path, might have lower labor costs,” he explains.
Lab-grown diamond producers are not out to price their diamonds as low as possible. “We look at market prices of natural mined diamonds, demand for diamonds and so on and adjust our prices accordingly,” says Franklin. Other producers, including Hill from Washington Diamonds, also use natural diamond pricing benchmarks to set their prices. “The discount to mined diamonds is more than enough for all of our customers to be having business and strong growth with lab-grown diamonds,” Hill notes.
The next step in lab-growing diamond technology is about making diamonds bigger, faster and whiter. For HPHT diamonds, the challenge remains removing nitrogen from the growing process. Today, while HPHT technology is capable of producing a D color diamond, it is painstakingly slow and the size is limited. The average HPHT diamond color revolves around G color in VS clarity. Likewise for CVD diamonds, the next frontier is achieving faster growth for bigger sizes while maintaining good color and clarity. “We now have greater control on the gases, and we have been increasing the size of the growth platform,” says McGuire from Scio Diamond. “We will continue to push the limits of technology for whiter goods and better cost efficiency,” he concludes.Article from the Rapaport Magazine - April 2016. To subscribe click here.
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