Introduction
Chemical Vapour Deposition (CVD) is the second major method for producing gem-quality lab-grown diamonds. Where HPHT mimics the Earth's mantle — extreme pressure, extreme temperature, a molten metal environment — CVD takes a fundamentally different approach. It grows diamond from gas, at low pressure, in a plasma chamber, depositing carbon atoms one layer at a time onto a flat diamond seed.
The result is a different kind of crystal. CVD diamonds grow as flat plates rather than the rounded forms of HPHT growth. They contain no metallic inclusions. They tend to be cleaner under magnification, but they carry their own set of identifying characteristics — growth striations, specific defect centres, and a brown colour in their as-grown state that typically requires post-growth treatment to correct.
CVD has become the dominant production method for gem-quality lab-grown diamonds. India leads production, followed by the United States, China, and Israel.
The Growth Process
The Chamber
A CVD reactor is a vacuum chamber, typically cylindrical, equipped with a microwave generator. The chamber contains a flat diamond seed plate — a thin slice of diamond that serves as the substrate onto which new diamond will grow. Multiple seeds can be placed in the chamber simultaneously, allowing several crystals to grow in parallel.
Gas Chemistry
The chamber is filled with a mixture of gases:
- Methane (CH₄) — the carbon source, typically comprising 1–5 % of the gas mixture
- Hydrogen (H₂) — the dominant gas, serving multiple roles: it etches away non-diamond carbon (graphite, amorphous carbon) that forms alongside diamond, and it stabilises the growing diamond surface
Additional gases may be introduced for specific purposes. A small amount of nitrogen can accelerate growth (at the cost of colour). Oxygen-containing gases can improve crystal quality.
Plasma Activation
Microwave energy ionises the gas mixture into a plasma — a superheated state of matter where molecules are broken into their component atoms and ions. The plasma temperature near the seed surface reaches 700–1,000 °C, far below the temperatures used in HPHT but sufficient to drive the chemistry of diamond growth.
Within the plasma, methane molecules dissociate into carbon atoms and hydrogen. The freed carbon atoms drift downward and bond to the diamond seed surface, extending the crystal lattice one atomic layer at a time. Meanwhile, hydrogen atoms continuously etch away any non-diamond carbon that forms, ensuring that only the diamond phase accumulates.
Growth Conditions
- Temperature: 700–1,000 °C at the seed surface
- Pressure: 10–200 Torr (far lower than HPHT's 5–6 GPa — roughly atmospheric pressure or below)
- Growth rate: Up to 0.2 mm per hour under optimised conditions
- Growth time: 3–4 weeks for gem-quality crystals, often with stop-and-start cycles to manage crystal quality
The low-pressure, gas-phase environment is what gives CVD its distinct advantages and limitations. No metal flux means no metallic inclusions. But the layer-by-layer growth produces different kinds of defects.
Crystal Growth and Morphology
CVD diamonds grow as tabular crystals — flat plates that extend laterally from the seed rather than growing outward in all directions like HPHT or natural octahedral crystals. The crystal expands upward from the seed surface, layer by layer, with each new layer inheriting the crystallographic orientation of the one below it.
This growth pattern creates characteristic internal features. Under cross-polarized light or fluorescence imaging, CVD diamonds often show banding or striations parallel to the growth direction — visible layers recording the stop-and-start cycles and varying growth conditions during production.
The tabular morphology also means that CVD rough is typically cut differently from HPHT or natural rough. The flat growth habit suits certain cutting orientations and may influence the shapes and proportions available at a given carat weight.
Characteristic Features
Growth Striations
The most recognisable internal feature of CVD diamonds under magnification or fluorescence imaging. These appear as fine parallel lines or bands within the crystal, reflecting the layered growth process. They are visible under DiamondView fluorescence imaging and sometimes under standard microscopy.
Silicon Vacancy Defect (SiV⁻)
A defect centre specific to CVD diamonds, caused by silicon contamination from the chamber walls or seed holder. The SiV⁻ centre produces a characteristic doublet in photoluminescence spectroscopy at 736.6 and 736.9 nm. This spectroscopic signature is one of the most reliable identification markers for CVD origin.
As-Grown Colour
Most CVD diamonds grown at commercially viable speeds are brown in their as-grown state. The brown colour results from vacancy clusters and other lattice defects introduced by rapid growth. Slowing the growth rate improves colour but reduces production efficiency. The commercial solution is post-growth HPHT treatment — see Post-Growth Processing.
Clean Internal Appearance
Because CVD growth occurs in a gas environment with no metal flux, CVD diamonds contain no metallic inclusions. Many are remarkably clean under magnification — sometimes suspiciously so for natural diamond grading standards, where some degree of inclusion is expected. The absence of both metallic inclusions and natural mineral inclusions (garnet, olivine) is itself a diagnostic indicator.
Scale and Geography
India is the dominant producer of gem-quality CVD diamonds, supported by an established diamond cutting and polishing infrastructure. The United States, China, and Israel are also significant production centres.
CVD reactor technology has advanced rapidly. Single crystals exceeding 13.5 carats have been reported, and multiple stones can be grown simultaneously in a single chamber run. Production costs continue to fall as chamber designs improve and growth rates increase.
Frequently Asked Questions
How long does it take to grow a CVD diamond?
A typical gem-quality CVD crystal takes 3–4 weeks to grow, often with stop-and-start cycles to manage crystal quality. Growth rate under optimised conditions reaches up to 0.2 mm per hour.
Why are most CVD diamonds brown as-grown?
Rapid growth creates vacancy clusters and lattice defects that absorb light and produce a brown tint. Approximately 80 % of CVD diamonds submitted to the GIA have undergone post-growth HPHT treatment to remove this brown colour.
How can you tell a CVD diamond from a natural one?
Growth striations visible under fluorescence imaging, the SiV⁻ spectroscopic signature at 736 nm, the absence of natural mineral inclusions, and distinct fluorescence patterns under DiamondView imaging are the primary indicators. See Spectroscopy Overview and Fluorescence Imaging.
Can CVD diamonds be as large as natural diamonds?
CVD technology has produced single crystals exceeding 13.5 carats. While most production targets the 1–3 ct range for commercial efficiency, the size ceiling continues to rise.
Summary
CVD diamond growth deposits carbon from a methane plasma onto a flat diamond seed at low pressure and moderate temperature — a fundamentally different process from HPHT's high-pressure metal flux method. The result is a tabular crystal that grows layer by layer, often inclusion-free but bearing distinctive features: growth striations, the SiV⁻ defect centre, and brown as-grown colour that typically requires post-growth treatment. CVD has become the dominant method for gem-quality lab-grown diamond production, led by India, with crystal sizes, growth rates, and production efficiency all continuing to improve.