Lab grown diamonds are diamonds, primarily constructed of carbon atoms in a crystallised lattice structure, made in a lab/controlled environment. Lab diamonds are chemically, physically and optically identical to mined diamonds. There are two primary methods of creating lab diamonds: Chemical Vapour Deposition (CVD) and High Pressure High Temperature (HPHT). In this article we will address the differences, pros and cons and which produces the better lab diamond.
How are HPHT diamonds grown?
High Pressure, High Temperature replicates the natural conditions of diamonds formed deep within the earth by subjecting carbon to extreme temperature and pressure.
HPHT diamonds are grown by placing a seed inside an insulated growth cell, which is then placed at the centre of a hydraulic press and subject to extreme pressure and temperature, typically around 2000 degrees celsius and 1 million psi. Carbon, in the form of graphite, melts under these conditions and atom by atom crystallizes onto the diamond seed. The time it takes for the diamond to grow depends on the desired size and quality.
HPHT diamonds grow in a both octahedral and cubic shape, with growth occurring in 14 directions.
How are CVD diamonds grown?
CVD diamonds are grown by placing a lab diamond seed into the reactor, gas is removed and hydrocarbon is added. Microwave energy breaks apart the hydrocarbon molecules. Atom by atom, the crystal structure grows vertically in the shape of a cube. CVD diamonds have one growth direction.
The number of diamonds grown depends on how large the CVD reactor is and how many diamond seeds are placed inside.
CVD Vs HPHT - what are the differences:
1. Colour:
HPHT diamonds can come in a range of colours from near colourless, blue, brown or yellow hues depending on the trace elements used during growth.
CVD diamonds tend to come out with a brown/grey hue which can be post growth treated with HPHT to make them colourless.
In some cases, post growth treating a brown CVD diamond can make it appear worse.
2. Control Over the Process
HPHT: Once growth starts, the HPHT press is usually not stopped. Manufacturers rely on precise calculations and formulas, as the process cannot be visually monitored.
CVD: The reactors have a viewport, allowing manufacturers to monitor the growth. This provides greater control but stopping and restarting the process can introduce defects due to the changing conditions.
3. Colour distribution/zoning:
HPHT diamonds can display geometric colour zoning which is an uneven distribution of colour throughout the diamond. This is caused by the elements concentrating at different levels while the diamond formed.
Naturally formed diamonds can also have colour zoning but it will not contain a geometric pattern like HPHT diamonds.
CVD diamonds tend to have an even colour distribution because the gasses (hydrogen and methane) spread evenly during the diamond formation.
4. Fluorescence:
Both HPHT and CVD diamonds can exhibit fluorescence, which causes them to glow when exposed to UV (ultra violet) light and stop glowing once this light source is removed.
Fluorescence is mentioned in the grading report.
Phosphorescence, when the diamond continues to glow for seconds or even minutes after the light source has been removed, is unique to some HPHT diamonds due to boron impurities and is not mentioned in grading reports.
5. CVD Vs HPHT Cost:
CVD diamonds cost less to produce than HPHT diamonds. CVD diamonds require smaller equipment, versus HPHT which requires large reactors weighing 70 tonnes each.
HPHT can't grow as many diamonds compared to CVD so it has a lower production efficiency
6. Inclusions:
Both CVD and HPHT diamonds can contain inclusions, these inclusions can vary in size and type, various factors influence these.
HPHT diamonds may contain metallic inclusions, due to metals used during the growth entering the diamond. If the metallic inclusions are large enough, the diamond can be picked up by magnets.
CVD diamonds do not develop metallic inclusions, instead they can develop dark pinpoint inclusions, such as graphite inclusions.
7. Strain:
CVD diamonds display strain patterns, the intensity varies.
HPHT diamonds tend to not have strain patterns because the pressure applied during growth is even, if they do, they are very minor.
8. Passing a diamond tester:
Diamonds are thermal conductors and electrical insulators.
Old basic diamond testers used to test for thermal conductivity, how quickly heat travels through the stone. However, with moissanite entering the market, which is also a great thermal conductor (not as good as diamonds), moissanite was passing basic diamond testers. Diamond testers have had to evolve to test for thermal and electrical conductivity.
Due to trace elements, diamonds can have different electrical properties to what is typical, boron if concentrated enough, can have a notable effect on the electrical conductivity of a diamond. giving off the results of synthetic moissanite.
Synthetic moissanite can also test as diamonds as it has been refined over time to have lower electrical conductivity.
CVD Vs HPHT: What to look out for:
When shopping for lab-grown diamonds, it’s important to know what to look out for in each type:
CVD: Greys, Browns, Strains and Striations
HPHT: Blues, Browns, Phosphorescence
CVD vs HPHT which is better?
There is no definitive answer to which method is superior. Both CVD and HPHT have their strengths and weaknesses. While both methods can produce stunning, high-quality diamonds, they can also yield lower-quality stones if poor materials or growth catalysts are used.
