Lab-Grown Diamond Identification (8.04)
At this writing lab-grown diamonds have had a dramatic affect on the jewelry industry in both having downward price pressure on natural diamonds and making fine jewelry more affordable. It is a reality that must be taken seriously by keeping yourself informed and well versed in both value and identification.
Lab-grown diamond, the most common verbal description, is also known as synthetic, cultured, created and man-made diamond. In this very important section we will delve into identification by means of their characteristic features.
In the process of identification the creation methods are important to understand. The only two methods are high-pressure high-temperature (HPHT) and chemical vapor deposition (CVD).
Growth Method of High-Pressure High-Temperature (HPHT) Lab-Grown Diamonds
A diamond seed or very small fragment of a high quality lab-grown or natural diamond is placed in the chamber with a carbon source such as graphite along with the the metal catalyst, iron, cobalt, or nickel. Once the chamber is sealed and heated to at least 1,500° Celsius (2732° Fahrenheit), and the pressure has achieved 4.5-6.0 gigapascals (approximately 725,000 pounds per square inch), the carbon source will dissolve with the aid of the catalyst metal. The dissolved carbon atoms then crystalize around the diamond seed over weeks or months.
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Creating the substrate for CVD production. The substrate is the seed or platform that the crystal will grow from that is created from an HPHT seed making a flat diamond surface that will later be detached from the finished CVD rough crystal.
A type Ib (types discussed further in this subsection), HPHT single-crystal-diamond ranging in size of 3.5mm square and up to 12mm square, is cleaned and polished to make sure all contaminants have been removed, typically with hydrogen plasma etching.
The seed crystal is placed within the chamber of the CVD reactor and then sealed with carbon- rich methane gas and an even larger amount of hydrogen which removes non-diamond carbon, (graphite) in keeping the quality at an optimum. Other gases such as nitrogen can be introduced to create fancy colors or boron to produce blue diamonds.
The gas filled chamber is then heated to 900°–1200 °C (1652°-2192° F) using microwave energy creating a charged high-density plasma. Other plasma producing heating sources can be utilized such as high-intensity lamps or hot-filaments. The plasma can also be created by using radio frequency or direct current. This process strips the carbon and methyl atoms (methyl is created from the decomposition of the hydrocarbon gas), from some of their electrons creating an open electron shell allowing the precipitated radical carbon atoms to attach to the HPHT diamond seed. The methyl radical atom is crucial in the deliverance and new diamond carbon atom formation on the surface of the seed diamond.
This homoepitaxial growth which is the process of growing crystalline layers upon identical material. The growth process is carefully monitored and adjustments can be made in pressure, temperature, microwave intensity and gas mixture to optimize the quality and the growth speed. These adjustments can also be implemented for prevention of non-single-crystal areas (polycrystalline PCD rims). New developments have created substrate holders and temperature control that allows further upward distance of growth.
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After several weeks of the substrate growth process the seed diamond must be separated from the new substrate. Traditionally this was done with a laser or diamond embedded sawing wheel both of which are slow and waste more material in the process.
The modern technique utilizes hydrogen ions which are implanted into the surface of the seed diamond at a specific depth with a hydrogen ion accelerator before the growth process begins. This creates a weakness on that plane whereas heat, gas or electrochemical methods will divide the seed diamond from its CVD counterpart. This is leaves a clean division with no unnecessary weight loss and a reusable seed.
Growing the CVD diamond on the substrate. These 10-20mm square substrates are placed within the vacuum chamber filled with methane and hydrogen gases. These gases are activated by microwaves and as a result the carbon atoms are released on to the substrates one layer at a time growing the cubical crystal straight up. This is why sometimes you’ll see the dark graphite in layers which has a non-metallic appearance versus the HPHT metallic inclusions left behind in their growth process.
Identification of Characteristics in Lab-Grown Diamonds
Laser Inscription: Microscopic marking: The inscription is a permanent, microscopic mark etched onto the girdle (the thin outer edge) of the diamond using a fine, precise laser beam. This inscription is traditionally placed on the girdle and is very small, so a loupe or gemologist microscope will be necessary to read it.
Diamond Types: Since the vast majority of lab-grown diamonds are type IIa (but not all), identification is critical. The two main categories are type I and type II which are subdivided into these five types: Type IaA, Type IaB, Type Ib, Type IIa, and Type IIb. Using an infrared spectrometer the amount of impurities are measured within the carbon crystal lattice on the atomic level. Natural diamonds are often mixes of Type Ia and Ib (IaB), which is identified by their infrared absorption spectral profile.
Absorption Properties: Using a spectrophotometer (including the spectrometer integrated into this instrument), we can determine absorption properties.
Ultraviolet absorption: From the high transparency of type IIa diamonds ultraviolet (UV) to the high absorption of UV (and infrared) of the type I diamonds across the spectrum.
Infrared (IR) absorption: The aggregate formations of nitrogen atoms within the diamond lattice create characteristic infrared absorption features. Infrared spectroscopy is a primary method for determining a diamond's type based on these unique "fingerprint" absorption bands.
Visible absorption: The configuration of nitrogen atoms also causes absorption in the visible spectrum, which can give the diamond a yellow or brown tint. The absorption spectrum in the visible region depends on the specific type of nitrogen aggregates present.
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Type I Diamonds
Type I diamonds are characterized with the main impurity being 0.1% nitrogen. The absorption properties include ultraviolet wavelengths shorter than 320 nanometers. In addition infrared and visible absorption spectra are measured. These visible absorption properties are primarily caused by the presence and placement of nitrogen atoms within the diamond's lattice structure.
Type IaA diamonds have nitrogen atoms that are arranged in pairs which do not affect the color.
Type IaB diamonds have nitrogen atoms in large even-numbered arrangements causing a visible yellow to brown hue.
Type Ib are rare making up only 0.1% of natural diamonds, containing 500 ppm (parts per million). The atoms are arranged in throughout the crystal lattice in isolated areas causing the absorption of green and blue light resulting in the intense fancy yellows and sometimes brown.
Type II diamonds have no presence of measurable nitrogen atoms. Spectrometer absorption shows up in a different area of the infrared, and the ultraviolet reads below 225 nm, unlike Type I diamonds. The fluorescence characteristics also differ. The irregular-shaped mined crystals are generally large. It has been found the the type II diamonds were formed under a more extreme pressure for longer time periods compared to the type I.
Type IIa make up an estimated 1.8% of gem-quality diamonds. This type generally has no detectable impurities usually rendering them colorless with a higher thermal conductivity than it's type I counterpart. In the spectral analysis ultraviolet is down to 230 nm. Sometimes, while being forced to the surface, the extreme pressure can cause structural abnormalities leading to imperfections cause a variety of colors. Since they are type IIa diamonds the color can be lightened with the high-pressure high-temperature treatment.
Type IIb make up about .1% of all natural diamonds. Like the the type IIa diamonds, the type IIb has very low levels of nitrogen to undetectable. The major difference is the presence of detectable boron which causes an absorption of of the yellow, orange and red parts of the light spectrum rendering a visible color of light blue to grey. Blue to grey color hues can also occur in type 1a diamonds (as in the Argyle mines in Australia), and a very slight level of boron in a type IIb can render a diamond colorless. As little as 1 ppm of boron will also make the type IIb diamonds p-type which creates a semi-conductor due to the uncompensated electron holes it leaves behind. When analyzing the spectra it shows a sharp infrared absorption gradually increasing toward the red side of the visible spectrum.
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Not restricted to type are green diamonds, whose color is derived from exposure to varying quantities of ionizing radiation.
Most blue-gray diamonds coming from the Argyle mines of Australia are not of type IIb, but of Ia type; those diamonds contain large concentrations of defects and impurities (especially hydrogen and nitrogen) and the origin of their color is yet uncertain.
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The author highly recommends this article from GIA's Gems & Gemology Summer 2024 issue: https://www.gia.edu/gems-gemology/summer-2024-gia-update-on-laboratory-grown-diamonds
Magnetic properties in HPHT lab-grown diamonds may occur on some (not all) of this lab-grown diamond due to its presence of ferromagnetic nickel, iron and cobalt used as a solvent catalyst allowing the carbon to dissolve and re-crystalize. On a loose diamond a rare magnet is used on a smooth surface to test for attraction.
Inclusions in HPHT lab-grown diamonds will not be a common sight in the modern, updated lab-grown diamonds due to the vast improvements made over the years, although they will occasionally surface. These inclusion types will primarily include metallic flux inclusions which remnants of the carbon-dissolving catalysts such as iron, nickel and cobalt that are left behind. The amorphous, spherical or rod-like shapes appear opaque and metallic under magnification. With a large enough presence these inclusions may cause magnetic attraction.
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The rare dendritic inclusions in HPHT lab-grown diamonds, are shaped like trees or bushes left behind from the molten flux catalyst that is trapped within the crystal in the growth process. They occurs due to the fluctuations in the precisely controlled lab environment of temperature, pressure, or chemical composition.
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Inclusions in CVD lab-grown diamonds
Again, typically in the modern lab-grown diamonds due to years of improvements, SI1 and lower clarity grades are not a common sight, although they will surface from time to time.
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