How Lab-Grown Diamonds Are Made: CVD vs HPHT Guide

Understanding the process matters practically: some types of HPHT produce specific optical characteristics that show on an IGI certificate. And knowing the difference between CVD and HPHT helps you understand the certificate you receive.

What Lab-Grown Diamonds Actually Are

Before the process: what exactly is a lab-grown diamond?

A lab-grown diamond is a real diamond. Same chemical composition: pure carbon atoms in a cubic crystal lattice. Same hardness: Mohs 10 (hardest substance in nature). Same refractive index. Same density. Same optical properties.

The only difference is where it was made. A mined diamond formed 100-200 km below the Earth's surface over billions of years and was brought to the surface through volcanic activity. A lab-grown diamond was made in a controlled growth environment over days or weeks.

Gemological labs — GIA, IGI, and others — grade lab-grown diamonds on the same 4Cs scale as natural diamonds: cut, colour, clarity, and carat weight. IGI is the most common grading lab for lab-grown diamonds in the Canadian market.

Process 1: Chemical Vapor Deposition (CVD)

CVD is the most common method used for gem-quality lab-grown diamonds today.

How it works:

1. A thin diamond seed (a small slice of existing diamond, natural or lab-grown) is placed in a sealed vacuum chamber.
2. The chamber is filled with a carbon-rich gas mixture — typically methane (CH₄) and hydrogen (H₂).
3. The chamber is heated to approximately 700-1,000°C using microwave or radio-frequency energy.
4. The energy breaks down the gas molecules, releasing carbon atoms.
5. Carbon atoms deposit layer-by-layer onto the seed crystal, building up a new diamond.
6. Growth takes several weeks for a gem-quality stone.
7. The rough diamond is cut from the substrate and polished.

Characteristics of CVD diamonds:

• Typically Type IIa (very high purity, no nitrogen or boron impurities)
• Can produce a very high colour range — D-E-F is common in CVD production
• Growth occurs in flat layers, which is why many CVD diamonds are flat-grown
• May show faint brown or grey tints (called "post-growth color") that are removed by a subsequent HPHT treatment

The CVD + HPHT combination: Many CVD-grown diamonds undergo an additional HPHT treatment after CVD growth to remove residual colour tints. This is standard practice, not a defect.

Process 2: High Pressure High Temperature (HPHT)

HPHT was the original lab-grown diamond process, first successfully used by GE in 1954.

How it works:

1. A carbon source (graphite or carbon powder) and a small diamond seed are placed together in a metal catalyst (typically cobalt, nickel, or iron).
2. The assembly is placed inside a hydraulic press.
3. Extreme pressure (5-6 GPa — roughly 1.5 million pounds per square inch) and extreme heat (1,300-1,600°C) are applied.
4. The metal catalyst melts, facilitating carbon dissolution and recrystallization around the diamond seed.
5. As conditions are slowly changed, carbon deposits onto the seed, growing the diamond crystal.
6. Growth takes days to weeks depending on desired size.

Characteristics of HPHT diamonds:

• Often cubic octahedral crystal form (eight-sided crystal shape, different from CVD's flat plate growth)
• Can produce high colour but often contains more nitrogen (Type Ia or Ib) — which can produce yellow or brown tints in some stones
• The metal catalyst can sometimes leave metallic inclusions, visible on some IGI clarity grades
• Some HPHT diamonds show a distinctive growth sector pattern under polarized light

HPHT as a colour enhancement treatment: Separate from growing new diamonds, HPHT is also used as a treatment to improve the colour of existing lab-grown CVD diamonds. This isn't a newer process — it's a standard post-growth finishing step.

What This Means for Your IGI Certificate

An IGI certificate doesn't always specify CVD or HPHT — but some certificates list the growth method. What you will see:

• Cut, colour, clarity, carat: Graded identically to natural diamonds on the 4Cs scale
• "Lab-Grown" or "Laboratory-Grown": Clearly indicated — IGI uses different certificate colours for lab-grown (light blue) vs. natural (beige)
• Laser inscription: The girdle of the diamond is laser-inscribed with the IGI report number. This is how you verify the stone matches the certificate.
• Fluorescence: Some lab-grown diamonds, particularly HPHT, show strong fluorescence under UV light. This is noted on the certificate.
• Type classification (sometimes shown): Type IIa (typically CVD, very pure) vs. Type Ia/Ib (typically HPHT, may contain nitrogen)

Does the Growth Method Affect Quality?

For practical jewellery purposes: no, if the stone has been graded to a good colour and clarity level.

Both CVD and HPHT can produce excellent, colourless, eye-clean diamonds. Both processes produce stones that are graded identically. The quality of the finished stone is what matters — and that's captured in the IGI grade.

Where it can matter:

• Very high colour (D-E) diamonds are more commonly CVD because the process more readily produces colourless stones
• Budget stones at lower colour grades may be HPHT with nitrogen content
• Metallic inclusions in HPHT are rare but possible at lower clarity grades

CVD vs. HPHT at a Glance

Why Lux Jewels Specifies Lab-Grown Only

I founded Lux Jewels in 2007 and made the shift to exclusively lab-grown in 2015 for straightforward reasons: lab-grown diamonds are chemically and optically identical to mined, they're certified by the same grading labs, and they're available at a fraction of the price for the same quality grade. Since 2015, when I became the first Canadian jewellery specialist to commit exclusively to lab-grown, I've sourced both CVD and HPHT stones — choosing based on the grade and value for each client's specific requirements.

If you want to discuss which process has produced the best options for your specific budget and grade target, that's exactly what the consultation is for.

Book a free video consultation