Post-Growth Treatments in Lab-Grown Diamonds: The Hidden Quality Factors

If you are browsing lab-grown diamonds, you may notice that two stones with identical GIA or IGI grades (such as F color, VS1 clarity) can have notably different prices.

This price variation often relates to a hidden quality factor: whether the diamond underwent post-growth treatment. Because growing diamonds quickly in a laboratory often introduces color impurities or structural strain, many manufacturers use post-growth treatments to improve the stone’s final grade.

This detailed, objective guide explains how post-growth treatments work, why they affect pricing, and how to identify them on a grading certificate.

1. Why Do Lab Diamonds Need Post-Growth Treatment?

Growing diamonds in a laboratory is a rapid, delicate process. When Chemical Vapor Deposition (CVD) diamonds are grown quickly to maximize factory output, they often develop a brown tint. This brown coloration is caused by minor structural defects (vacancies) in the carbon lattice where atoms are missing.

To improve these lower-grade diamonds, manufacturers use post-growth treatments to repair the crystal structure, bringing the diamond's color into the highly desirable near-colorless (G-H) or colorless (D-F) range.

2. The HPHT Annealing Process

The most common post-growth treatment is HPHT annealing.

[ CVD Diamond with Brown Tint ] ➔ [ HPHT Annealing (Extreme Heat/Pressure) ] ➔ [ Repaired Carbon Lattice ] ➔ [ Colorless (D-F) Diamond ]

During this treatment, the newly grown CVD diamond is placed inside a High Pressure High Temperature press. The intense heat (typically over 1,800°C) and stabilizing pressure allow the misplaced carbon atoms to relax back into their proper positions. This process heals the structural vacancies and permanently erases the brown tint.

While HPHT annealing is safe and stable, untreated "as-grown" CVD diamonds are much harder to produce and require highly precise, slower growth cycles. Consequently, untreated stones command a premium in the market.

3. Checking the IGI and GIA Lab Diamond Certificate

To protect your purchase, you must review the grading report from gemological laboratories like the GIA or IGI. These reports will clearly disclose any post-growth treatments.

IGI Grading Reports

The International Gemological Institute (IGI) is highly transparent regarding treatments. If a diamond has undergone post-growth treatment, the IGI certificate will include a note under the "Comments" section at the bottom, stating:

"Post-growth treatment detected." or "May include post-growth treatment."

If the stone is completely untreated, the IGI report will state:

"As Grown - No indication of post-growth treatment."

GIA Grading Reports

The GIA also notes treatments on their laboratory-grown reports, using the comment:

"This is a man-made diamond... post-growth treatments may have been applied."

Quick Comparison Summary

Conclusion: Which is Better?

Understanding post-growth treatments helps you decide whether to prioritize absolute natural growth or pure value.

• Choose an "As Grown" Untreated Lab Diamond if: You value the technical feat of growing a highly pure diamond crystal without relying on secondary heat treatments, and you are comfortable paying a small premium for this extra level of quality.

• Choose a Treated Lab Diamond if: You are focused on securing the best possible visual appearance (color and clarity) for your budget, and you are happy to accept stable, laboratory-certified treatments that lower the stone's cost.