Growing Diamonds

Lab Grown Diamonds: Can they be the future of the Diamond industry?

INTRODUCTION

Diamonds: the symbol of love and rarity but also the cause of many brutal civil wars in several African countries, including Sierra Leone, as depicted in the movie Blood Diamond. Additionally, they cause huge environmental destruction due to irresponsible mining techniques resulting in acid mine drainage, contaminant leaching, air pollution (with a median CO2 footprint of 108.5Kg per carat) (1), rerouting of rivers, deforestation and soil erosion which leaves large areas of land impossible to farm and with no wildlife (2) . However, the profitability of these mines depends on the perceived rarity of diamonds; a myth fashioned by DeBeers (the company which for a long time controlled the production of most of the world’s diamonds) with their signature slogan ‘A diamond is forever’ used since 1948, in order to increase their price by making them artificially scarce (3) .

The technology and process to produce synthetic diamonds has existed since the 1940s. Due to the start of the Second World War, however, the project pioneered by General Electric (codenamed “Project Superpressure”) was initially postponed, succeeding years later in December 1954 to produce the first diamonds synthesised in a lab (4). The 1950s represented the start of large scale diamond manufacture which, due to them not being as high-quality as natural diamonds, were used for industrial purposes, including laser optics and healthcare. Gem-quality diamonds were first synthesised by researchers at General Electric (GE) in 1970, followed by other manufacturers in the mid 1980s (5) . But can these lab grown diamonds really replace a mined stone, uprooting the established $82billion diamond industry?

NATURAL DIAMONDS

Diamonds naturally form around 150200km deep in the Earth’s mantle where temperatures average 9001300oC and pressures range from 45 to 60 kilobars. In these conditions, carbon atoms are able to crystallise into regular arrays and form diamonds (6); a process that can take millions of years. A narrow vertical pipe of igneous rock called kimberlite, created by a highly explosive volcanic eruption, brings the diamonds closer to the surface where they can be found and mined. The kimberlite ore (which is then mechanically broken down, separating the diamonds from the rock) has a very low diamond content with roughly one-part diamond per million parts rock requiring large amounts or ore to be processed and yielding large volumes of waste.

Natural diamonds form an octahedral shape, with the diamond growing outward from the centre resulting in growth on all the faces, giving a concentric square pattern to the crystal.

Infrared spectroscopy (a process with uses infrared light and measures absorption, reflection and emission when used on a molecule) is the primary way of identifying different types of diamonds. In the 1930s, scientists began recognising that certain kinds of diamonds displayed similar properties; for example, a blue diamond often exhibited electrical conductivity. Based on differences in transparency to ultraviolet radiation and the fact that nitrogen was the principal chemical impurity in natural diamonds (which was discovered in 1959), they grouped diamonds into two main categories: Type 1 which contain nitrogen, and the less common Type 2 which does not. Both types have subcategories that further differ in their atomic structure. Type 1A and 1B vary in how the carbon and impurity atoms are arranged; the former has aggregated nitrogen atoms while the latter has a single nitrogen atom replacing a carbon atom. Type 2B diamonds contain a small amount of boron while Type 2A are chemically pure.

SYNTHETIC DIAMONDS

There are two main techniques for growing diamonds in the lab: the High-Pressure, High-Temperature (HPHT) method and the Chemical Vapour Deposition (CVD) method. The former was the initial process used by GE and works by mimicking the conditions that naturally occurring diamonds form under. The latter, a more modern and increasingly popular technology, uses lower pressures and temperatures than HPHT as well as cheaper equipment. As shown in the table opposite, the technique used to produce lab-grown diamonds influences their shapes and properties.

HIGH-PRESSURE, HIGH-TEMPERATURE (HPHT) METHOD

This process uses a small diamond fragment known as a seed crystal which acts as a diamond template and a special apparatus which is able to produce the very high pressures that diamonds need in order to form. The seed is placed in a capsule containing graphite or another suitable pure carbon alternative which is dissolved in a molten flux, made up of a metal catalyst such as iron, nickel or cobalt, lowering the temperature and pressure needed for diamond growth. As the seed is cooler, the carbon material migrates through the flux towards it and crystalizes on the surface forming a synthetic diamond crystal. This process is much faster than forming natural diamonds as it occurs over a period of several days to weeks to grow one or several different crystals.

In order to make sure that the diamonds synthesised are of a higher-purity, the growth time must be increased with greater control over pressure and temperature conditions, while nitrogen, which when included will make the diamonds a yellow colour, must be excluded from the growth environment. If blue diamonds are desired, boron is introduced whereas other colours, including pink and red, are obtained through postgrowth treatment involving radiation and heating.

HPHT synthetic diamonds are easily distinguishable from natural ones due to their differing internal growth patterns. While natural diamonds, as shown above, typically form octahedrons, the HPHT technology forms diamonds which have cubic faces. In some cases, the synthesised diamonds may have dark flux-metal (i.e. a metal solvent) inclusions or may exhibit phosphorescence which means that the material, having been exposed to ultraviolet light, will continue to glow even when the light source has been removed. Furthermore, diamonds that are grown with the HPHT method often have unique colour distribution, fluorescent zoning and graining patterns due to their crossshaped growth structure which are not found in natural diamonds. With more recent technological advancements in the HPHT method, scientists have now been able to produce colourless crystals up to a size of 10 carats and above.

CHEMICAL VAPOUR DEPOSITION (CVD) PROCESS

Similar to the HPHT process, chemical vapour deposition requires a diamond seed crystal. However, unlike the above method, the seed is placed in a vacuum chamber which is then filled with a hydrogen and carbon containing gas such as methane. A microwave beam, or another suitable source of energy, is used to break down the gas molecules in the chamber. The carbon atoms in the gas then diffuse towards the colder diamond seed. Over a period of weeks, crystallisation takes place resulting in the growth of a diamond. With this method, the exact number of diamonds grown can vary depending on the size of the chamber used and the number of initial seed plates placed in the chamber.

The diamonds synthesised by CVD have a tubular shape and are brown in colour (which is removed by a heat treatment known as HPHT annealing). They often have a rough edge of black graphite that is then cut away.

In the same way as the HPHT growth process, the CVD method also continues to improve due to technological advancements, resulting in larger sized of crystals with improved colour and clarity.

COMPAIRING LAB GROWN (HPHT AND CVD) AND NATURAL DIAMONDS

Despite the fact that natural and synthetic diamonds have the same chemical composition, their unique growing conditions contribute to them having specific features which allow scientists to identify by which method they came about. These are summarised in the following table and are described further below.

Colour distribution/zoning - During a diamond’s formation, any change in temperature or uneven incorporation of trace elements results in an irregular distribution of colour within a diamond. Natural diamonds tend to not have any colour zoning, however, when they do, they are not of a regular pattern. Coloured diamonds formed by the HPHT method usually present with a geometric colour zoning pattern as a result of how elements which contribute to the colour concentrate in the diamond during the formation process. On the other hand, CVD diamonds commonly have an even internal colouration.

Metallic flux inclusions and dark

pinpoint inclusions - Metallic flux inclusions appear black and opaque in transmitted light, but have a metallic shine in reflected light, are often present in HPHT synthetic diamonds. This is a consequence of the metals used as catalysts entering the diamond during growth; these flux metal alloys contain elements such as iron, nickel and cobalt which can result in the diamonds being able to be picked up with magnets if they contain a large amount of these metallic inclusions. In contrast, dark graphite or mineral inclusions can be found in CVD synthesised diamonds instead of metallic inclusions which mean they do not have a metallic shine to them. Natural diamonds on the other hand, have none of these inclusions.

Strain patterns. Stress, due to erupting from deep in the Earth’s mantle to the surface, can result in natural diamonds exhibiting a crosshatched or mosaic pattern of interference (or ‘strain’) which can be seen using polarising filters angled at 90-degrees to one another. Synthetic diamonds do not show the same pattern of interference while CVD diamonds can display banded interference patterns while HPHT diamonds have no strain patterns or weak banded ones due to them growing at nearly uniform pressure and are consequently not subjected to any stress.

Fluorescence - Synthetic diamonds differ to their natural counterparts due to differing fluorescence colours and patterns. For example, natural diamond fluorescence is stronger at long wavelength ultraviolet light while weaker at shorter wavelengths while the contrary is true for synthetic diamonds which also have distinctive colour patterns not present in natural diamonds. (7)