Post-Growth Treatments of Laboratory-Grown Diamonds

After growth is complete, a laboratory-grown diamond need not be in the condition in which it will reach the market. A producer may subsequently alter its color or optical defects through heat, high pressure, irradiation, or a combination of processes.

Four questions must therefore always be separated: what the material is, the method by which it was grown, what it was like as-grown, and what was done after growth.

HPHT Growth Is Not HPHT Treatment

HPHT growth creates a new diamond crystal in a high-pressure cell. HPHT treatment processes a preexisting diamond at high temperature and pressure to alter its defect structure and often its color. Treatment does not add a new diamond layer or change the growth origin.

The description CVD-grown, HPHT-treated is therefore entirely logical. Such a stone remains a CVD-grown diamond.

Why CVD Material Is Often Subsequently Annealed

Rapid or nitrogen-assisted CVD growth may produce a brown component associated with a complex defect structure. High-temperature processing can reorganize some of those defects and reduce the brown color.

GIA reported that, in its population of CVD diamonds since 2020, approximately 80% of the CVD specimens examined showed signs of post-growth processing, predominantly HPHT annealing. This figure describes GIA submissions, not total global production or every CVD product on the market.

HPHT treatment of CVD diamonds is conducted at temperatures higher than those typical of HPHT growth itself, often above approximately 1600 °C in the GIA process contexts described. The outcome depends on the initial defect structure; the same process need not produce the same color change in every stone.

LPHT Annealing

A CVD diamond can also be heat treated at low pressure, for example in a vacuum or inert atmosphere. Such a low-pressure, high-temperature (LPHT) process is not “weaker HPHT”; it is a different thermodynamic regime. It can alter parts of the vacancy and nitrogen-vacancy systems and affect color, but interpretation must be tied to the specific sample and documented procedure.

Irradiation and Subsequent Annealing

Irradiation can create vacancies in the lattice. Subsequent thermal annealing allows them to move and combine with other defects. Optical centers such as NV or H3 may thus form or intensify, depending on the starting material and treatment sequence.

The final color is therefore not a simple function of the “radiation dose.” It depends on the initial atomic type, concentration of impurities, defects, energy and type of radiation, spatial distribution of damage, and the temperature and duration of subsequent annealing.

Multistep sequences involving HPHT processing, irradiation, and lower-temperature annealing have been documented in laboratory-grown fancy-color diamonds. Such examples demonstrate the capabilities of the processes, but they must not be turned into a universal recipe for a particular color.

As-Grown and Post-Growth Signals May Coexist

Subsequent processing does not necessarily erase every trace of growth. CVD growth striations may remain visible after HPHT treatment. Some spectroscopic centers may disappear or change intensity, while others may appear. It is precisely the combination of a preserved growth pattern and an altered spectral signature that often makes it possible to reconstruct the history.

One example is the 596/597 nm system, which is often associated with as-grown CVD material and may disappear after HPHT processing. H2, H3, NV, and other centers may change depending on the process. None of them alone represents a complete treatment chronology.

Coatings and Clarity Treatments Are Separate Problems

A surface coating is not the same as changing the defect structure of the entire crystal. A film or surface layer can alter perceived color without rearranging the internal lattice in the same manner as annealing or irradiation.

Fracture filling and laser drilling belong to the area of clarity enhancement and are addressed in later treatment chapters. They should not automatically be associated with laboratory-grown origin: the same treatment concept may be applied to different categories of material.

Stability Is Assessed by Process

It is not professional to say that “all treatments are stable” or “all treatments are unstable.” Internal defects reorganized at high temperature may be stable during normal wear, while a surface coating may be more sensitive to abrasion or service procedures. Jewelry repair may involve heat, lasers, chemicals, or ultrasonic cleaning, so the jeweler must know the documented treatment history when it is relevant.

Chapters 57–63 cover treatments in detail; the essential logic here is: growth method, post-growth treatment, and the stone’s current condition are three separate pieces of information.

Reconstructing Treatment History

A professional workflow begins by confirming the diamond material and growth method. Next, color, growth zones, luminescence, and atomic type are documented, with UV-Vis-NIR and PL data added as needed. The surface is examined for coatings or other interventions.

Only then are possible treatment sequences compared. If multiple histories explain the findings equally well, the conclusion must remain limited. Undetermined is not a failure when the body of evidence does not permit a stronger claim.

Chapter Summary

• The as-grown state and final market state of a laboratory-grown diamond may differ.
• HPHT growth and HPHT treatment are entirely different processes.
• CVD-grown + HPHT-treated remains a CVD-grown diamond.
• HPHT annealing is often used to reduce the brown component of CVD material.
• GIA’s post-growth rate of approximately 80 percent applies to its CVD submission population since 2020, not to global production.
• LPHT annealing occurs at low pressure and must not be equated with the HPHT process.
• Irradiation can create vacancies, while subsequent annealing alters their arrangement and optical centers.
• Final color is not a reliable “recipe” for reconstructing treatment.
• Growth traces may survive subsequent processing.
• A single PL or absorption center is not a complete treatment history.
• Surface coatings and clarity enhancement are separate categories.
• The strongest professional conclusion must sometimes remain undetermined.

[VISUAL 52.1: Growth origin versus post-growth treatment—HPHT-grown, CVD-grown, and CVD-grown/HPHT-treated]

[VISUAL 52.2: Representative treatment pathways—annealing, irradiation, and combined sequences]

[VISUAL 52.3: As-grown CVD and HPHT-treated CVD—preserved growth striations with altered spectroscopic centers]

[VISUAL 52.4: Treatment evidence matrix—growth architecture, color, FTIR, UV-Vis-NIR, PL, and surface]