Which feature is commonly used to identify HPHT lab-grown diamonds?

• A. Gas inclusions
• B. Absence of inclusions
• C. Metallic inclusions or flux, and color zoning or growth striations
• D. Fluorescence patterns under UV light

Answer: (C)

The key idea is that HPHT lab-grown diamonds carry predictable internal features from the growth process that natural stones typically do not show. In HPHT synthesis, a metal catalyst such as nickel or another flux is used to dissolve carbon and promote crystal formation under extreme pressure and temperature. Fragments of this metal catalyst or flux can get trapped inside the growing diamond, producing metallic inclusions or residual flux that are a telltale sign of HPHT origin. Alongside this, the way the crystal forms in distinct growth layers leads to color zoning or growth striations—bands or stripes within the diamond that reflect changes in growth conditions over time. The combination of metallic inclusions or flux and visible color zoning or growth striations is characteristic of HPHT diamonds and helps distinguish them from other types.

Gas inclusions, absence of inclusions, and fluorescence patterns under UV light are not reliable identifiers on their own. Gas inclusions can appear in various diamonds, and many natural stones also show fluorescence patterns that are not unique to HPHT origins; thus these features are not as diagnostic as metallic inclusions/flux and growth zoning.