Process differences between CVD and HPHT

CVD and HPHT are currently the most popular cultivation diamond production processes. What are their differences, and as consumers, which production process should we choose for cultivating diamonds?

 

The world of cultivated diamonds is far more complex than natural diamonds. According to data from the International Council of Cultivated Diamonds (IGDA), the global cultivated diamond market has formed a dual technology dominant pattern - CVD occupies 72% of the high-end jewelry market share (>3ct), while HPHT controls 58% of light luxury products (<1ct). This differentiation stems from the distinct growth mechanisms of the two. Choosing high-quality cultivated diamonds requires a deep understanding of the differences between CVD and HPHT.

Comparison of Technical Principles and Processes

1.1 Chemical Vapor Deposition (CVD) method

Process mechanism: Based on plasma enhanced chemical vapor deposition technology, carbon containing gas (such as methane) is introduced into a vacuum reaction chamber, and the gas is dissociated into carbon atoms through microwave energy. Type Ib natural diamond flakes are used as seed crystals for layer by layer epitaxial growth.

• Growth rate: 0.5-3 μ m/h (laboratory grade equipment), single crystal growth cycle is about 3-4 weeks.

 

Crystal characteristics:

• Growth mode: preferentially grow along the (100) crystal direction;
• Purity: Type IIa diamonds account for over 85% (natural diamonds only 2%), without nitrogen and other metal impurities, and the purity generally reaches VVS level or above;
• Optical performance: The color is highly consistent with natural diamonds, and the diamond luster is natural;
• Defect control: No metal cladding, magnetic or phosphorescent phenomena;
• Color expression: The intrinsic color is mostly F-J color, which requires high-temperature annealing (1800 ℃) for color modification treatment;
• Growth size: Typical size can reach 10ct or more.

 

1.2 High temperature and high pressure method (HPHT)

• Process mechanism: A six sided top press machine is used to simulate the mantle environment (5.5-6GPa, 1300-1600 ℃), and Fe Ni Co alloy flux is used to catalyze the phase transition of carbon source (high-purity graphite).
• Growth rate: 0.2-0.5 carats per hour.
• Crystal characteristics:
• Crystal morphology: cubic octahedral polymer;
• Impurity characteristics: Type Ib is the main type, containing trace amounts of nitrogen (<10ppm) and metal inclusions, with a metal inclusion content of about 100-500ppm;
• Color expression: About 68% of HPHT diamonds have a blue tone (due to boron impurities), and the primary color can reach D-E color level;
• Special phenomenon: Some batches exhibit phosphorescence reactions (emitting light in the dark)
• Mainstream products: 1-3ct

 

 

3.1 Identification of CVD color modification process

• Photoluminescence spectrum: annealed CVD diamond exhibits a characteristic peak of silicon vacancy center (Si-V ⁻) at 737nm;
• Cathodoluminescence: presenting characteristic growth bands;
• DiamondView： Display a typical layered growth structure.

 

3.2 HPHT feature detection

• Magnetic sensitivity: slight displacement can be observed using a 0.5T permanent magnet;
• X-ray morphology: displays a cross shaped stress distribution;
• Infrared spectroscopy: Detected a nickel related defect peak at 1282cm ⁻¹.

 

 

Consumer Decision Model

4.1 Quality Requirement Orientation

Wedding level selection:

• CVD selection: Type IIa accounts for more than 85%, with high clarity and a large grain size of 2 carats or more (relatively low cost). However, when selecting color grades, it is important to avoid choosing diamonds that have undergone color modification treatment and confirm that there is no "Post growth treatment" note in the certificate;
• HPHT Preferred: Suitable for selecting high-grade D-E colors (no need to worry about color modification), VVS-VS for clarity (less metal wrapping, less magnetism), and preferably marked as "None" in the Clarity Characteristics column of the IGI certificate.

 

Light luxury level selection:

• Suggest choosing 0.5-1ct HPHT D-E color vs. clarity;
• Avoid batches with magnetic reactions (magnetic susceptibility>5 × 10 ⁻⁵ SI).

 

The current cultivation of the diamond market shows a clear technological differentiation: CVD has a 72% market share in the jewelry grade market above 3ct (according to Bain data in 2023), while HPHT maintains a 58% market share in the jewelry field below 1ct. Consumers are advised to focus on the distribution of crystal defects and spectral characteristic parameters based on GIA/IGI detection data, rather than relying solely on traditional 4C systems. 

 

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